It is observed by an excellent optician and writer on the microscope that “the manner in which an object is lighted is second in importance only to the excellence of the glass through which it is seen. In the investigating of any new or unknown object it should be viewed in turns by every description of light, direct and oblique, as a transparent object and as an opaque object, with strong and with faint light, with large angular pencils and with small angular pencils, thrown in all possible directions. Every change will probably develop some new fact in reference to the structure of the object.” These remarks are so true that it is not too much to say that the power and perfection of the best modem lenses cannot be correctly estimated or fully appreciated unless employed in conjunction with the best modes of illumination; nor can the best methods of illumination be properly tested without the best lenses. But, in proportion as these optical inventions, like most other contrivances, approach perfection, so do the difficulty and care necessary in using them increase; and hence, to secure their full advantage, it becomes the more necessary to possess a certain amount of knowledge both of their construction and their action.
In the judging of optical instruments it is of importance that appropriate objects should be examined—namely, such as have upon them the most delicate, though distinct markings.
I know of no microscopic specimen better adapted for testing the excellency both of lenses and condensers than the one now generally in use for this purpose—the Pleurosigma angulatum,
I shall frequently have occasion in the following remarks to allude to Gillett’s condenser and to Wenham’s paraboloid, but as these excellent contrivances have been in use for some time, and therefore are generally known, it would be superfluous in me to describe their several parts, or to do more than simply to name them.
Nothing can show the advantages of the improved method of illumination better than a careful examination of the object just named, first as illuminated in the ordinary way, and then as illuminated in the improved one; afterwards contrasting the appearances which it presents under these kinds of illumination.
If the Pleurosigma angulatum be examined with a lens of l-8th inch focus, and 150 ° angular aperture, by good daylight reflected upon it by a plane or concave mirror in the ordinary way, little more than the mere outline of the object will be visible. Nor will any advantage be gained by increasing the magnifying power of the microscope by the employment of the deeper eye-pieces. On thè contrary, these are of more harm than benefit, in consequence of the diminution of light which they occasion. If, now, Gillett’s condenser be substituted for the mirror, and the light be admitted only through the four or five smaller apertures of the revolving diaphragm of the condenser, so that the central rays only of the pencil of light pass through the object, no dots or lines will be seen upon it, but its appearance will be the same as when the mirror was used; nor will the deeper eye-pieces be of any use. Of course I am speaking of the condenser when properly centered and adjusted. The non-appearance of markings on the Pleurosigma under this kind of illumination is differently explained. That it is not due to a deficiency of light is evident from the following experiment. If one of these apertures be stopped, and the diaphragm be so placed that only a small quantity of light can pass obliquely through the condenser by the next hole, distinct parallel lines are made apparent, although the field of view is much darker than before. And that it is not in this instance caused by an excess of light, as imagined by some microscropists, is certain from the following fact. Turn the revolving diaphragm so that the pencil of light passing through the condenser is just sufficient to render the markings on this object visible; afterwards bring successively the larger apertures under the condenser, and it will be seen that the marking, in the place of becoming less apparent as the diameter of the pencil of light increases, becomes more so.
From these facts it is obvious that the appearance or nonappearance of lines on the Pleurosigma is altogether independent of the quantity of light, and due only to the direction in which the rays are made to fall upon this object. As it has been shown that the rays nearly perpendicular, called direct rays in contradistinction to the oblique ones, are of no use in the illumination of the object in question, and as its marking is rendered perfectly distinct by oblique light, it is evident that the most proper illumination in this case is that in which the central part of the pencil of light is cut off by a stop from the object, and only the oblique rays allowed to pass through it. These ends are fully attained by Mr. Gillett’s condenser: and, as by this contrivance the oblique rays can be thrown equally on all sides of the minutest particles, shadows are prevented; and markings, which, when illuminated by oblique light only on one side, appear as lines, are in this way resolved into their component dots.
It now remains to show in what manner oblique light acts in developing structures which cannot be seen by nearly perpendicular rays. I may observe that it is not a question of degrees of distinctness that I am considering, but the fact that parts, which are perfectly distinct when examined by one kind of illumination, are totally invisible when examined by another kind.
The explanation of this fact seems to be deducible from the following considerations.
Suppose a part to be made up of two substances intimately connected, though distinct from each other, and both definitely arranged, whose refractive powers differ so little that they cannot be distinguished from one another under the microscope by the slight difference in their refraction of the light passing through them. This is, I believe, the condition of the Pleurosigma angulatum and other objects of a similar kind. Now, if the light, by any kind of illumination, can be prevented passing through one of these substances—the one having the greater refractive power—whilst it passes freely through the other, they will then become perfectly distinguishable, the one appearing dark, the other light. This is what seems to take place when such objects as the one in question are illuminated by oblique light; for a ray of light cannot pass out of a denser into a rarer medium if the angle of incidence exceed a certain limit, and this limit is different in substances of different refractive powers.’ Thus all rays incident on water, at an angle greater than 48° 36′, having to pass from it into air will not be refracted, but reflected. In the same way, all rays incident on glass, at an angle greater than 41° 49′, and passing from it into air, will not be refracted, but suffer total reflection. Hence, applying these facts to the Pleurosigma, I think that it admits of but little doubt that the dots appear dark only because the light beneath falling upon them at an angle greater than that at which all refraction ceases, and total reflection begins, cannot be transmitted, and hence these dots are seen as opaque bodies intercepting the passage of the light to the eye; whilst, on the contrary, the other material, having a lower refractive power, and therefore allowing all the oblique rays incident upon it at the same angle to pass through it, will necessarily appear bright.
The correctness of this conclusion will appear more probable when it is recollected that these two substances are distinguishable not by the one refracting the light differently to the other, but by the fact of one refracting it, and the other not; the former appearing bright and transparent, the latter dark and opaque.
As respects the non-appearance of the markings under direct illumination, it may be observed that, as the rays in this instance may nearly all be supposed to be incident upon the object atan angle less than that at which refraction ceases, they would be refracted by both substances nearly in the same degree, and therefore each would appear to be transparent, and the whole almost homogeneous.
These few facts show that, when all such objects as the Plewrosigma are illuminated by oblique rays, they must be examined by lenses which admit a large pencil of light, that is, have a large angle of aperture, in order that an allowance may be made for the diminished quantity of light which penetrates the object and enters the eye, in consequence of the total reflection from the lower surface of the dots.
Having considered the class of objects best fitted to display the effects of oblique illumination, I will now consider those which are best seen by light passing through them almost perpendicularly.
Although oblique light answers so well in the instances I have adduced, there are some structures and objects for which it is totally unfit, and which can only be successfully examined by rays passing through them almost perpendicularly, that is, by direct light.
Amongst this class of objects are all those which strongly refract light, either from their density or spherical figure, as, for instance, most recent structures, either animal or vegetable, these consisting chiefly of corpuscles, and highly-refractive particles of various forms and sizes.
The reason why such objects cannot be seen when illuminated by rays falling upon them very obliquely, but are distinct when illuminated by those which fall upon them almost perpendicularly, will, I think, be apparent from a few considerations respecting the undulatory theory of light. According to this theory, the phenomena of refraction are due to vibrations produced in an elastic medium occupying the intervals between the particles of all transparent substances by a force proceeding from a luminous body; and the elasticity of this medium is less in proportion to the refractive power of every transparent substance; or, in other words, the greater the refractive power of any substance the greater also will be the force required to excite undulations in the less elastic medium diffused through it. Hence, as the effect of the same forcé acting upon a resisting medium is proportional to the direction in which it acts, being at its maximum when the line of the force is perpendicular to that of the resistance, and at its minimum when the angle of inclination is upon the point of vanishing, it must be clear, that light falling obliquely upon a transparent surface will exert less power in producing the effects of refraction than if it fell perpendicularly; so that when the rays of light fall very obliquely upon a highlyrefractive substance, their effect will be too feeble to excite its condensed vibratory medium into undulations, and therefore the rays will simply pass by it, producing at its border the effects of interference of light.
This is precisely what takes place when oblique light is employed to illuminate objects possessing a very high refractive power; whilst rays falling nearly perpendicularly upon the same objects, and thus acting upon them in a direction the most favourable for producing the effects of refraction, penetrate, as it were, their substance, and render their structure apparent in all its detail.
Though there are these two classes of objects, one requiring for their illumination oblique rays, and the other nearly perpendicular ones, yet. the majority of compound organs require both kinds of light. Many of them are structures which, though they may appear most beautiful under direct illumination, will, by oblique light, be made to reveal something in their composition which would have remained concealed without this light.
Structures, if examined properly, should be subjected to a kind of microscopic analysis, in order that nothing in their composition may be overlooked.
Having shown some of the advantages of the present methods of illuminating microscopic objects, I will consider some defects in these methods, which have been in a great measure overlooked, and also the best way of obviating them.
This will be best done by carefully observing the effects of different modes of illumination upon those microscopic objects whose precise form and optical properties are known.
For this purpose I will first describe the appearances presented by microscopic globules of mercury of different sizes when illuminated by Gillett’s condenser and Wenham’s paraboloid.
Such globules can easily be obtained by condensing the vapour of boiling mercury upon a piece of glass, and then causing some of the particles to run together, with the point of a needle.
If one of these globules, about l-300th of an inch in diameter, be examined by a lens of half-inch focus, and illuminated by Gillett’s condenser, all rays coming from other sources being completely cut off, and the light admitted only through the smallest aperture of the revolving diaphragm, it will present, when the margin is in focus, a circular, darkish surface with an obscure ill-defined light in the centre; but when the nearest surface is brought into focus, the central spot of light will become distinct and well defined. If the diaphragm be revolved, so as to bring under the condenser the larger apertures, the size of the central spots of light will gradually increase in proportion to the size of the apertures. If, now, one of the stops be brought under the centre of the condenser there will be seen on this globule, in the middle of the illuminated circular space, a distinct image of the stop, which can be recognised by the cross-bar which joins the circular disk to the edge of the opening; and these can further be shown to be the image of the parts just mentioned by revolving the condenser, when they will be seen to move and to change their direction and position according to that of the condenser. If there be several globules of different sizes in the field of view, every one of them will have an image of the stop upon it. Of course in proportion as the globules are more minute, the images will be less recognizable, and on those about 1-1000th of an inch in diameter they can be distinguished only as a very minute circular spot with a dark point in its centre. Globules much smaller than these present only a minute point of light in their centre; and the smaller, those about 1-15000th of an inch, appear entirely dark. However, when higher magnifying powers are employed, an image of the stop can be distinguished on globules l-4800th of an inch in diameter. If any of the globules have been a little compressed by the piece of thin glass placed upon them, to keep them from dust, so that their spherical figure is destroyed, no image will be formed on them; but the flattened central space, when the stop is under the condenser, will be faintly illuminated; if the place of the stop, however, be occupied by one of the apertures, it will be very brightly illuminated. Such are the appearances of globules of mercury under the higher power of the microscope. But if they are examined by a lens of lower power—one-inch focus, with one of the larger stops under the condenser—they will appear on a dark ground, as when illuminated by Wenham’s paraboloid. There will be a welldefined ring of light around each globule, and at its centre an image of a stop; the only difference in their appearance as illuminated by these two instruments being this, that when the latter is employed, the light is a little brighter, and that in the place of the image of a stop in the centre of the globules there is a very distinct one at the end of the paraboloid, and of the cross-bar placed within the tube to support the central disk, which can be seen to move and change its direction when the instrument is made to revolve.
