I Received lately a letter from Count Franc Castracane, in which he desires me to forward to you the following remarks on the Woodward observations reported in the ‘Quart. Journ. Mic. Sci.’ for July, 1866.

“I quite agree,” he says in the beginning, “with Mr. S. J. Woodward, that for microphotographical uses one may obtain nearly the same results with any undecomposed ray of light which has been transmitted through a solution of ammonio-sulphate of copper ; and I am equally persuaded of the usefulness of microscopical objectives ready made to obtain the coincidence of the action with the visual forms.

The usefulness of a light modified by traversing a coloured medium has been pointed out on other occasions ; and M. Bruster has lately suggested to illuminate the microscope with alcoholic light, saturated with chlorine of sodium. Such is the opinion, too, of Dr. Noiterrier (p. 180 and following of his very useful book ‘La Photographie appliquée aux Recherches Microscopiques,’ lately published in Paris by Baillière et fils.)

From the application of decomposed light to photography, I expected, indeed, a great advantage, which I did not obtain, as I tried to take any microscopical object illuminated with a violet ray resulting from the sun-light decomposed by an enormous prism, and sulphuretted carbon, which I had constructed on purpose. Nevertheless, I cannot but insist upon the usefulness of illumination with decomposed and vigorously microchromatic light, which may be obtained only with a good prism, seeing that the cobalt glass and the solution of ammonio-sulphate of copper admits with the violet some part of other rays, so that we can never obtain an absolute correction of the chromatic aberration, which it is known cannot be obtained by any combination of lenses, no matter how perfect they may be.

A proof amongst others of the efficacy of this illumination has been to resolve by it very easily the thirty groups of Nobert’s test-lines, which Mr. Norman of Hull has been so kind as to lend to me, which experiment has been witnessed, amongst others, by the well-known director of the Astronomical Observatory of the Roman College, P. Angelo Secchi.

“A considerable augmentation of power in the microscope for the use of such an illumination would afford an easy way to decide the question which micrographers are still debating about the true form of the minute structural details of diatoms ; first among them is the Pleurosigma angulatum, which earliest microscopists have deservedly chosen to test the power of their instruments. It is true that the material improvement which the microscope has obtained these last fifteen years has superseded this old test object. Still we may be allowed to observe that the assumed easiness in resolving the details of pleurosigma must be understood for the oblique, not for the direct and central illumination, especially when the preparation is made in Canada balsam.

“Whatever the direction of the light may be, and not with standing the great improvements which the microscopical objectives have received, the mode of explaining those very minute forms which adorn the surfaces of diatoms is still at variance. Schiff, Schultze, Schact, and Hartnach, amid the German—Wallich, Wenham, and Carpenter amongst the English, do not agree -with themselves on this subject. They began noticing on pleurosigma some very minute striæ which present themselves in a particular direction under the influence of an oblique illumination ; then, changing the course of the light, they observed another system of striæ. Then the opinion of some writers who, having noticed successively three different systems of striæ, two oblique and one direct, concluded they ought to be disposed in different planes. But more perfect objectives by connection and immersion,showing the three systems of striæ simultaneously, caused the first judgment to be rejected, and acknowledged that they were placed on the same level. The greatest difficulty they met with was to determine the shape of the areola limited by the different directions of the striæ. Some believed they were square, assuming that the transversal system of striæ was nothing but an illusion caused by aberration of spheriority. Schact considers them as hexagons, each side of them being the basis of a small equilateral triangle. Quekett, following Wenham, who succeeded in obtaining an image of P. angulatum, increased to 15,000, and another of P. formosum to 35,000 diameters, recognised and described the structure of these pleurosigmata as a series of hexagonal spaces by which the surface of the valves is partitioned.

“.Now, Mr. Woodward tells us that Wenham, abandoning his previous judgment, has acknowledged that the conformation of the markings is circular. Thus the hexagonal appearance in Wenham’s photographs would be nothing, according to Jabez Hogg, but ‘an exaggerated imperfection produced by an error of foix in his lenses.’

“These discordant views of the most celebrated micro- graphers encouraged me in trying to get a most possibly accurate idea on the subject, for the which purpose I examined most carefully the photographs of P. angulatum taken by myself and by others, too, under different degrees of power, giving my especial attention to the negatives on glass, which, it is well known, present a greater nicety of details than positives. And availing myself of the monochromatic illumination, which I always employ in testing the most difficult object, I felt convinced that the hexagonal form is the only true structural element of the surface of P. angulatum, seeing that by a direct observation with light decomposed, the striæ present themselves always bended (in zigzag) and never straight, and that but in three directions—one transverse, and two oblique.

“Besides all this, I must confess that I cannot understand how an object not perfectly in focus, or excessively magnified, may produce the illusion of a circular changed in any other angular form, whilst the contrary, it is obvious, succeeds whenever the vision of an object is less distinct, say for its being too far off, or for interposition of mistiness, or for any other cause.

“This is my simple manner of viewing the thing. However, I should always be glad if anybody would show me that I am mistaken. At any rate I wish they would try the monochromatic light, which I feel confident they will find useful to decide this as well as many other difficult points.”

So far Count Castracane’s letter on the subject. If there is in my rough translation any technical or other error, I hope you will correct it.—Yours very truly, PROFESSOR JOSEPH GAZLIARDI, Cardiff.

—Mr. John Bockett sends us a photograph of his method of mounting and using a microscope lamp. A pillar upon a foot carries a glass lamp with a reflector behind it, and a condensing lens in front. The reflector is about three and a half inches in diameter, and the bull’s-eye condenser about two inches in diameter, and placed a little within the focus of the reflector. A shade is also provided. We have long used and recommended the addition of a silver reflector behind a lamp. It not only economises light, but for many purposes improves its quality, as objects may be illuminated almost entirely by the reflected light when the wick is turned low, and thus the glare of the direct flame is avoided. Mr. Bockett burns Belmontine, which gives a whiter light than paraffine.—Intellectual Observer.

A Mechanical Finger for the Microscope

This is the name which Mr. H. L. Smith, of Kenyon College, U.S., has given to a very ingenious mechanical appliance, which will prove a boon to those microscopists who are engaged in the study of minute hard structures. Since the mere description of Mr. Smith’s invention occupies nearly three pages of ‘Sillimann’s American Journal of Science ‘(No. 123), we must refer our readers to this source for details. The instrument seems likely to be extremely useful in delicate manipulation, since it can be made to move about in every direction over the stage, and thus to convey minute objects from one part of the field to another—and this, too, with the greatest precision, and in the most gradual manner.—Lancet.

Transmission of Slides by Post

—At the meeting of the Quekett Microscopical Club, held November 23rd, Mr. M. C. Cooke called attention to this subject, on account of the large proportion of broken slides which had come under his observation during the past two years. Sometimes he had received a dozen slides per week from as many different individuals, and insufficient packing was the rule, good packing the exception. Many persons only enveloped their slide in stiff paper, some in cardboard, and then enclosed them in their letters : such slides were invariably broken. Others sent slides in thin cardboard boxes, or wrapped in cotton-wool or wadding, and afterwards in cardboard : these were generally broken. Others again enclosed slides between thin strips of wood with small blocks, or corks at the ends or angles : these seldom travelled safely. The most successful mode of packing proved to be, either to enclose the slides (if more than one) in a small deal box ; or, if single, to transmit them in the black paper cases sold by opticians. If these cases are folded in a sheet of paper, in such a manner that about two inches of the paper extends beyond one end of the case, and the postage stamps are affixed to this free end, there will be no risk of damage from the obliterating stamp. It was suggested that additional security from local stamping would be given by pasting black paper round that portion of the package which contained the slide, and the address written on the free end of the paper. This would be the only white portion, on which consequently all stamping must be performed. A number of boxes and other contrivances for the transmission of slides, and which had passed through the ordeal of a journey, were exhibited ; some of which, in their shattered contents, gave evidence of failure.

M. Enlenstein’s Series of Diatomaceæ

—Those of our readers who are more especially interested in the study of Diatoms, will be pleased to learn that M. Th. Eulenstein, of Stutgard, who is well known as one of the most active investigators of the subject, has undertaken the publication of two distinct series of specimens of Diatomaceæ. One series will consist of AUTHENTIC and ORIGINAL specimens ; and it is intended to facilitate the identification of the numerous species established by foreign authors. The uncertainty of nomenclature which has pervaded all the writings on this subject since the works of Ehrenberg and Kiitzing is entirely due to a want of accurate knowledge of these specimens, which M. Eulenstein has spared no pains to obtain for the present purpose.

Simultaneously with, but perfectly distinct from this series, M. Eulenstein intends also to publish another series, which will form, as it were, a STANDARD collection of the various types of the Diatomaceæ, and will contain typical representatives of nearly all the known genera, recent and fossil.

Each series, as we learn from the prospectus, will be issued in five parts, each part containing one hundred species. The first part of the first-mentioned series will consist chiefly of specimens selected from the herbarium of Professor Kiitzing, and will explain many critical species established by that author in his ‘Bacillaria ‘and ‘Species Algarum.’ The subsequent parts will contain original specimens illustrating the works of Ehrenberg, Heilberg, Grunnow, Rabenhorst, and others. Besides the numerous new and rare forms which will be found in this series, it will furnish systematists with a correct index to many species hitherto misunderstood, and therefore constitute an indispensable part of a very scientific collection of Diatoms.

We understand that the specimens will be carefully prepared dry or in balsam, and mounted on thin slides of the usual dimensions used in this country (3″ × 1″) ; and to each specimen will be affixed a label with the original name, locality, &c., whilst a separate list of synonyms, with critical notes, will be published with Part V.

It is hoped that the First Part of each series will appear in the early part of, and that the entire publication may be concluded within the year.

The number of collections belonging to the first mentioned series will necessarily be extremely limited ; but it is to be hoped that the London Microscopic Society will be the depository of one of them. Those of the second series would appear to be almost indispensable for all real students of the Diatomaceæ, and we can only wish that M. Eulenstein may find that the pains and trouble he has bestowed upon the formation and dissemination of these collections may be properly appreciated.

We have been given to understand that besides Mr. Pritchard, Dr. L. Beale and Mr. Roper will be ready to afford any further information respecting M. Eulenstein’s undertaking that may be required.

[Prospectuses may be obtained, and the Collections ordered, at Messrs. R. and J. BECK’S, 31, Comhill, E.C.]