Notes and Correspondence

Some time ago, at one of the meetings of the Microscopical Society, the model or perhaps the incomplete stage itself of a microscope was exhibited, in which Mr. King, of Bristol, if I remember rightly, had applied a magnet for the purpose of retaining a soft iron object-bearer, and at the same time of allowing it to be moved about in all directions with great smoothness and facility. I am not aware whether the inventor ever carried the design further into effect, but as the idea struck me as highly ingenious, and capable of very useful application, I have endeavoured to carry it out in the following simple manner. The contrivance, it should be remarked, can only be applied to a simple stage, that is to say, to a stage consisting of a single plate. The magnetic force is contained in two semicircular magnets of hard steel (fig 1)which are fixed on the under side of the stage, one on either side of the opening, with whose size the curve of the magnets should of course be made to correspond. The force is conveyed to the upper side of the stage by means of four soft iron pegs passing through four holes drilled in the stage, at points as near as may be corresponding with the four poles of the magnets, as at o, o, o, o (fig. 1). The iron pegs should slightly exceed the thickness of the stage in length, and should have a thin flattened head (fig. 2).

When inserted in the holes, into which they should fit closely, the magnets are screwed down upon the heads so as to be in close contact with them. All “that is now requisite to be done is to see that the ends of the pegs, which project through the stage, are precisely in the same plane, and raised about a hair’s breadth-above the surface of the stage, so as not to interfere with its level when used in other ways, and yet, at the same time, to prevent the object-bearer from coming in contact with the stage itself, by which its free movement would be interfered with. The object-bearer, which is made of thin soft iron, may be of any form or dimensions ; that which I have found useful, is of the shape shown in fig. 3.

The ends of the bearer being made to project beyond the sides of the stage, and being tapered off, serve as handles by which it may be most conveniently moved in any direction, with one or both hands, and as readily, if not more so, as by the usual screw-heads or lever of a compound stage.

In the case of a live-box all that is necessary is to have the plate, Upon which the upright part is fixed, made of soft iron instead of brass, to do away with the need of any other bearer. It is very important that the under surface of the iron object-bearers should be ground true and made very smooth, which may be done easily enough with a water of Ayr stone and fine emery-paper.

The advantages of this stage are the following : —

1. That the stage may be made very thin.

2. The universality of the movements, and the ease and simplicity of their execution by one or both hands.

3. Its trifling cost.

In conclusion, I would merely remark, that whatever credit is due for the original idea of a magnetic stage to the microscope belongs, as I believe, to Mr. King, of Bristol, and that it is only in following out the suggestion thus derived from him that I was led to the simple contrivance above described, in the execution of which I was assisted by Mr. Hudson, optician in Greenwich.—G. BUSK.

NOTE.—Since writing the above I have been kindly favoured by Mr. King, No. 1, Denmark-street, Bristol, with an account and sketch of his contrivance. It is more complicated, rather, than the one above described ; but is very ingenious, and, perhaps, if carried out practically by him, which has not yet been done, might, in some cases, be superior. It has the disadvantage, however, of not allowing the stage to be so thin as my contrivance does.—G. B.

It cannot be denied that one meaning of the word “Unscheinbarkeit “is “un-sightliness.” This is beyond all doubt ; and it may be admitted that in certain cases, the word might have the meaning attributed to it by Dr. Gregory (Q. J. Mic. Sc. vol. II. p. 201), although, perhaps, such meaning would be better expressed by “Unsichtbarkeit,” or “Unbemerkbarkeit.” But in the place where Fresenius uses the word, he is speaking not of the organs of Fungi, but of the tribe generally, to which the attribute of “want of obvious perceptibility” certainly does not belong. Moreover, the difficulty of studying Fungi, owing to their minuteness and the delicacy of their organs of fructification, is specially referred to by Fresenius, in the very next line, and it is not, therefore, probable, that he intended to use the word “Unscheinbarkeit “in a sense which would make the next sentence but one of mere tautology. Again, a little further on, the author says, that the causes he has mentioned may account for the neglect of the study of Fungi by those persons whose main object is, that their collections of plants should have a striking, neat, or elegant appearance. I think I have said sufficient to show that Fresenius did not use the word to express “want of obvious perceptibility.” “Unsightliness ”may be rather a strong expression, but we have no other single English word which would at all convey the author’s meaning. It might be paraphrased by “insignificance of appearance,” “meanness of appearance;’ or “want of power to captivate the eye.”—The Reviewer of Fresenius’ Mycology.

In feeding infusorial animalcules with carmine, one very great disadvantage presents itself, viz., the field of view becomes crowded with the dark particles of carmine, by which the object is hidden from correct observation, and the eye embarrassed ; another is, the length of time often required to render the gastric organs visible, owing to the slow imbibition of the carmine.

These difficulties, I think, may be obviated by the use of the red pigment which lines the cornea of the common housefly. I have tried it for two years, and, without an exception, in every case, found it devoured with avidity by the carnivorous animalcules, and from its being capable of such minute division, the field was left almost as clear as before the addition of it. The ciliary vibrations were perfectly distinct, and a beautiful bright red speedily made its appearance in the internal organs of the smallest animalcule present, while, in some cases, the larger crustacean Daphniœ, &c., appeared as if their blood had become coloured with it.

If you consider these simple suggestions of any service to your readers, they are perfectly at your disposal.—Thos. c. White, 65 Warwick Street, Pimlico.

In the course of a recent examination of the deposits of some of our tidal har-bours, the following species of Diatomaceæ have come under my notice ; and as they have not been figured, as far as I am aware, in any former publication, I annex camera drawings, with a short description of each, the insertion of which may prove interesting to some of the readers of your valuable Journal.

Triceratium armatum, n. s. (Fig. 1.)—Frustules large, with straight or slightly convex sides ; angles produced into long horn-like processes, with rounded extremities; cellular structure minute, partially radiating towards the sides and angles ; six or more spinous processes projecting from the surface of the valve.

I have seen three specimens of this fine Triceratium, sent me by my friend, F. Okeden, Esq., from Neyland and other localities, near Haverfordwest. It approaches nearly to the T. tridactylum, Ehr., of Mr. Brightwell, from Petersburg, Virginia,^* but differs in the size and form of the angles, and in wanting the siliceous plate that extends beyond the sides of that species. Professor Bailey has described a form from the same locality, with four lateral spines, which he named T. spinosum ; but, from his description and figure in Silliman’s Journal,^† I cannot satisfactorily identify it with the Neyland specimens, I therefore venture to apply the specific name of armatum. The spinous processes are very similar to those which occur on some species of Eupodiscus.

Triceratium comtum? Ehr. (Fig. 2.)—Sides straight or slightly convex, with a row of cells projecting above the margin of the valve ; the horn-like processes at the angles short and obtuse ; cellular structure large.

Specimens of this species were sent me, along with the preceding, from the same deposit, and appear to agree very closely with the figure given by Mr. Brightwell in the first volume, of the Microscopical Journal,^* The cellular markings are as large as in T. favus, and I am rather doubtful whether it may not be a young form of that species ; but the length of the angular processes and fringe-like row of cells at the margin appear to give it a distinctive character. It has not hitherto, I believe, been figured as British.

Doryphora? elegans, n. s. (Fig. 3.)—Valves obovate, divided p. 3 by a median line, with transverse striæ, disposed ‘‘in curves, concentric with the extremities ; the interspaces occupied by minute cells.

This pretty little species I met with in some mud from Pembroke harbour ; in form and structure it approaches the Natal specimens described as Euphyllodium spathulatum by Mr. Sbadbolt; ^† but I have lately heard from Mr. Brightwell that similar forms were previously referred by Mr. Smith to the genus Doryphora, and as his absence from England precludes me from submitting the specimens for his determination, I adopt his generic appellation. Until living specimens are obtained it must be considered a doubtful form ; but if stipitate, it would be included in the second sub-tribe of Mr. Smith’s Synopsis; and in structure would be nearer Doryphora, or Rhaphoneis of Kutzing, than any other genus. The structure of the valve is exactly similar to that of the Natal specimens, but differs in being smaller and less ovate. All the figures are drawn to a scale of 400 diameters.—F. C. S. ROPER.

Since writing the preceding note I have received the following description of the habitat of the two species of Triceratium, and I have reason to believe that the same deposits will yield several other curious forms. “The first specimen of this species (T. armatum), I found in a sample of the alluvial mud deposit at Neyland, which is a Creek of Milford Haven. The mud was obtained from a depth of twenty feet, with a boring tool constructed for the purpose ; I have since found it in a surface gathering from the same locality, and it also occurs in the upper deposits of the River Cleddon, within a mile of Haverfordwest. The other species of Triceratium occurs rather abundantly in the gathering from Neyland at twenty feet depth.”—FITZMAURICE OKEDEN, C.E.

I shall feel greatly obliged if you will insert in the next number of the ‘Quarterly Journal of Microscopic Science’ a few observations on Mr. Wenham’s last paper on Microscopic Illumination, in reply to some strictures made by him on a communication of mine upon the same subject.

I have no wish to impute to Mr. Wenham any other than the best motives, and I trust in the remarks which I am about to make he will give me credit for the same.

My principal object in the communication which you were so kind as to publish on the subject of microscopic illumination was to detail certain facts, the accuracy of which Mr. W. admits, designed to show the advantages and disadvantages of the present methods of illuminating transparent objects. The explanations which accompanied them I considered only to be of secondary importance, and though they appeared to me the most probable, yet I was ready to renounce them if they should be shown to be incorrect, or if reasons more probable should be advanced. Now in both these points Mr. W. has in my opinion failed, for on a most careful perusal of his paper I am unable to discover that he has either proved my explanations to be in any respect untrue, or advanced more feasible ones in their place.

As the explanation I have given of the cause of certain appearances presented by the Pleurosigma angulatum, under a particular kind of illumination, is merely the application of an optical fact, I see no reason for changing this opinion until it is shown either that I have misapplied this fact, or that what I have taken to be a universal law is not so. If I understand Mr. Wenham aright, he seems to me to deny the universal application of this law, “stating that light when incident at any degree of obliquity on diaphanous refracting bodies, with parallel sides, cannot be made to suffer total reflection either externally or internally.” Now, as in this assertion there is so much ambiguity and complexity, and as it differs so much from the simple manner in which I have always seen the law enunciated, I cannot give up that which is universally admitted as a fact for what I cannot clearly comprehend, and therefore I must beg to retain my opinion until Mt. Wenham has made good this assertion. In the application I have made of the law in question parallelism or non-parallelism has nothing to do with the subject, as the reflection referred to in my paper is supposed to be from one surface only, namely, from that on which the rays are incident. I am well aware that all refracting bodies are too thick to have only one surface, but this forms no reason for complieating the enunciation of the law, the refraction or reflection taking place at each surface, requiring only a separate and independent observation. However, if Mr. Wenham can prove by experiment that this fact is not universal I will give up my point, and in the mean time I am curious to know what the relation of the angle of incidence will be found to be to the angle of refraction, in the case of light passing from a denser into a rarer medium, after the complement of the angle of refraction has vanished.

Mr. Wenham next observes that “attempts have sometimes been made to draw the undulatory theory of light into the subject of microscopic illumination, but without any substantial reason, as it has in reality very little or nothing to do with it.” By this assertion, which is sufficiently dogmatical, I understand that the author means that the undulatory theory may have a little to do with microscopic illumination, but that, in fact, that little is so very inconsiderable as to deserve to be looked upon as nothing. Now this assertion cannot be correct, for as at present there are only two ways known of accounting for the phenomena of light, and these so dissimilar that if one be right the other is wrong. Hence, if the undulatory hypothesis be the correct one, it must apply as well to the facts connected with illumination as to the general facts of optics, and therefore if it apply in one instance to microscopic illumination, though in the least possible degree which Mr. Wenham seems to admit, it must be equally applicable to all.

With respect to Mr. Wenham’s comments upon the globules of mercury, where he states that “he does not consider that a globule of this substance, as being strictly opaque is at all suited for testing an illumination intended for transparent objects,” I may observe that this remark is unnecessary, as Mr. Wenham must have seen that these globules were never intended by me as a test of an illumination suited for transparent objects, but were merely employed to show some of the disadvantages of condensers, which purpose they answer extremely well by enabling us to illustrate a fact, which to my knowlege was not before demonstrated, namely, that in lenses of a short focus a great portion of the rays emanating from the source of light is reflected upon the object by the lenses composing the object-glass. Now these bodies being in all respects well adapted for illustrating and establishing this fact, and that in the most simple way, were I think very properly employed. But I did not stop here, the same effect was shown in my paper to be produced by transparent objects, though less in degree, that depending upon their form and density, and thus in the latter objects the light transmitted through them was shown to blend with that which they reflect, and so to produce a degree of confusion which, I maintained, ought to be taken into account, and allowed for, as far as possible, in the examination of all transparent objects.

This blending of transmitted and reflected rays, proceeding from the source of light, is greater when condensers are used than when only a plane mirror is employed, and therefore will be considered, I have no doubt, by all excepting Mr. Wenham, as an objection to these instruments, though, this being known and duly allowed for, the objection will be but trifling. Mr. Wenham says, “he cannot call to mind any ordinary object in which the reflection alluded to above takes place to such an extent as to create false appearances.” If Mr. W. will take the trouble to look at globules of air in glycerine, illuminated by means of his own instrument, he will see it for the first time. These globules thus circumstanced have certainly a most unnatural appearance, and would never be taken to be globules of atmospheric air. All other bodies partaking of the spherical form when examined in a fluid of a different refractive power, will exhibit the same fact. Oil globules in glycerine do not show it, but in water they do. See my paper. Lastly, Mr. Wenham objects to the employment of globules of mercury as a means of disproving the radiated light theory, stating “that he does not see that it all affects the question, simply because it is one of those few substances that is incapable of radiating light.” In reference to this part, I may observe that these objects were not employed with the positive intention suggested in this remark, but were simply referred to as a satisfactory means of showing that all the appearances attributed by Mr. Wenham to radiated light are explicable on the common principles of reflection, and thus I consider that they have an important bearing upon the question ; for it must be admitted that if these facts allow of an easy and obvious explanation upon long-established principles, there can be no necessity to invent new theories to explain them.

When I first became acquainted with Mr. Wenham’s condenser, which I acknowledge to be a very beautiful instrument, whose utility will rather be enhanced than abridged by any observations which I have made upon its mode of action, and when I first read his paper upon its use and construction, I was in favour of his hypothesis, and of the term “radiated light,” chiefly because the facts are so represented in his paper as to leave no other way of accounting for them, but on finding that all these facts admitted of an easy explanation upon well-known principles, I renounced his theory of radiated light, and if I can judge anything from a note in Mr. Wenham’s last paper, in which he evinces great dissatisfaction with the term “radiated light,” as “not being perhaps philosophically correct,” I am strongly of opinion that he either has or soon will follow my example.—George Rainey, St. Thomas’s Hospital.

I have, in my possession, a specimen of the spiral vessel of the rhubarb, given to me by a friend, but although I have viewed it with a low power of great excellence (Smith and Beck’s 2-3rd), yet I have never been able to see it satisfactorily with clearness and precision. This induced me to look at it with the polarizing apparatus, and when the Nichol’s prisms are turned so as entirely to darken the field, I was much pleased to see the spiral vessel beautifully illuminated and the spiral perfectly distinct. I was induced to try a “selenite stage,” in which the tints are violet and yellow in the alternate quarter revolutions of the polarizer, and the effect was not only exceedingly beautiful, but also very instructive, as the spiral exhibits, when the field is deep blue or violet-coloured, a most beautiful crimson, and is very clearly distinguishable from the investing membrane, thus affording an example of the utility of polarized light in certain circumstances.—G. Hunt, Birmingham.

The following Diatomaceæ were discovered in the recent United States Exploring Expedition, under Capt. Wilkes. The list here given is in the order of their geographical distribution.

A. largo collection of marine Algæ from New Zealand was examined, but no Diatomaceæ could be detected adhering to them.

Those marked thus (*) are believed to be new, and have been described. —Professor J. W. Bailey, in Proceedings of the Academy of Natural Sciences of Philadelphia, Oct., 1853.

Professor Wheatstone, the eminent physicist, in connexion with his remarks upon the value of the binocular microscope, in the July number of the ‘London Microscopical Journal,’ suggests that the monocular microscope may be made to give match stereoscopic pictures, by successively changing the inclination of the axis of the objective and ocular to the stage holding the object. This plan, though not easily made applicable to microscopes of the present construction, must, I think, give excellent results with the low powers, say with the two inch and inch objectives, and possibly with the half inch. But with the higher powers of large angle of aperture, the close proximity of the front surface of the objective to the thin glass cover of the objects totally precludes its being put in practice.

The method described below may be readily adapted to any microscope, at an expense comparatively trifling ; it is applicable to every grade of objective ; and upon fair trial I find it to give satisfactory results.

Behind, and close to the objective, insert an isosceles glass prism, say a half or a quarter inch equilateral or rectangular prism, adjustable for position, and capable of being inclined at pleasure any required number of degrees, on a central axis transverse to the axis of the ocular and objective, said axis being parallel to the polished faces of the prism. When the hypothenuse or reflecting surface of the prism is made coincident in direction with the axis of the microscope, the position of the prism being appropriate, the light travelling from the objective to the ocular will suffer reflection in its transit through the prism ; but the appearance and position of the field, except its reversal in one direction, will be essentially the same as if no prism were there. By inclining the prism a little, other objects are brought into view, as though the slide containing them were moved. If now, the slide be re-adjusted, so as to restore the field as at first, the objects will be seen from a different point of view, and will therefore wear a modified appearance.

The mode of proceeding is as follows : two good successive views of the same object are to be obtained, between which there must be a difference of inclination of the prism, say from four to eight or nine degrees, according to the depth of stereoscopy desired. In each instance, the principal object is brought to the centre of the field, by adjusting the position of the slide. In each instance, a careful camera lucida drawing is to be made, or a photographic impression taken ; which, when properly viewed, each by an eye, will be found to coalesce into a single image, manifesting the fine stereoscopic effect, which characterizes the image seen through the binocular microscope.—Professor Riddell, New Orleans Medical and Surgical Journal.

Several communications on micrometers have appeared in the ‘Microscopical Journal,’ and I am induced to address you on the subject from having particularly noticed the two following extracts of a letter from Mr. Jackson in your last Number:—

“The inquiries for a cheap form of microscope which 1 constantly hear, make me think that the difference between 4l. and 1l. for an adjunct to the instruments, would, in many instances, be a serious obstacle to the use of any means of minute measurement ; and it is with the view of placing these means within the reach of all observers that I have advocated ruled glass.

“To induce observers to make accurate measurements, which is the aim both of H. C. K. and myself, it is not sufficient to place an instrument in their hands ; they must be taught to use it with little trouble.”

I think that the following method of using the camera lucida with a stage micrometer answers the requisites of cheapness, facility, and accuracy :—

Place a stage micrometer in the focus of a microscope ; adapt a camera lucida, and then accurately trace on a piece of card-board, one, two, three, or more of the divisions. Subdivide each division by tens or (if need be) by hundreds ; then place the object to be measured in the focus of the microscope, and observe, through the camera lucida, the number of divisions it extends over on the card-board. For instance : I have an object-glass and eye-piece which, with the length of tube in my microscope, magnify 500 diameters ; and on looking with these at a stage micrometer, with 200 divisions to the inch, I find that each division occupies inches on a cardboard placed underneath the camera lucida on the table. I mark one of these spaces on the card-board, and divide it into 25 parts ; that is, into tenths of an inch, and, consequently, each tenth of an inch on the card-board corresponds to the 5000th part of an inch of an object in the focus of the microscope. Obviously, also, if instead of card-board, I use a slip of ruled glass, with a hundred divisions to the inch, each division will then correspond to the 50,000th part of an inch.

For convenience of calculation, it is desirable that each division of the micrometer should coincide with the lines of inches, or large fractional parts of an inch, on the card-board ; and this is easily effected when the microscope is furnished with a draw-tube ; but when the latter is wanting, the same point may be gained by elevating the card-board on a book or some kind of stage; of course, always taking care that the distance of the camera lucida from the card-board should be precisely the same, when an object is to be measured, as it was when the divisions were marked on the card-board. The ease of this method, also, in accurately determining the magnifying power of any combination of lenses and eye-pieces he may happen to possess, will be evident to any one attempting to practise it. The cost of a camera lucida is very trifling, and there would be no need to purchase a stage micrometer, if one could be borrowed for a short time, since a piece of card-board, once accurately marked^* in the above manner, would supersede its further use.—Henry Coles, Hammersmith.