will mention some particulars to be attended to without which the microscope would be altogether useless.

In the first place, it is absolutely necessary to illuminate strongly the objects we wish to observe. These objects being hardly ever luminous of themselves, emit directly very few rays; and only a small number of these are admitted into the microscope, on account of the very small opening which must necessarily be given to the object-lens. If then we confine ourselves to this small degree of light, the image will be so faint, that we can hardly ever perceive it, however little it may be magnified. On this account we illuminate the object strongly, by throwing upon it, from a slightly concave mirror, the ordinary light of the sky, or that of a lamp condensed by means of a converging glass. If the object is opaque, we illuminate it in this manner by light from above; but if it is transparent, we generally receive the light from below. I say, generally, because there are some cases in which it is best to have the light directed otherwise. When, for instance, we wish to observe the divisions of the micrometer, in order to determine the magnifying power, we never see them better than by an oblique reflection; then they are delineated in black on the glass plate. There are some of these micrometers which contain 900 visible marks in the space of a French line or of an English inch.

197. Another indispensable precaution is to place diaphragms in the interior of the instrument, to limit the field of view, and to exclude all that part of the image which is ill defined. For in all the preceding considerations, we have supposed the incidences and emergences infinitely small. They are not so in reality, and they are the less so, according as we give to lenses a greater opening. Hence the concentration of the rays in a single focus, the regular formation of the image, its perfect similarity to the object, and all the other properties which exist under very small inclinations, are only approximations from which we deviate more and more, in proportion as the glass used has a greater aperture. Now we must deviate only just so much as to prevent vision from becoming too defective; and this we do, by limiting the field of view by diaphragms, differing in extent according as the case may require; and a very simple method, is to retrench from the image whatever might affect the distinctness of its outlines.

198. Dr Brewster has contrived a very ingenious method of employing the microscope for measuring the ratios of refraction in liquids, and a great number of substances partially solid, such as wax, gum elastic, &c., which are opaque in the mass, but which become translucid, or even transparent, when made sufficiently thin. For this purpose, we fix before the object lens, and nearly in contact with it, a very thin plate of glass with parallel faces; then we bring the microscope to the point of distance proper for seeing distinctly, through this system, the marks of the object-glass micrometer. This being done, we insert between the glass plate and the lens, a drop of the liquid whose refraction we wish to examine; or if it is a solid substance, we detach a very thin lamina and press it strongly between the glass plate and the lens, until it is moulded, as it were, upon the surface of the lens. This operation forms, with the substance pressed, a true divergent meniscus, whose anterior surface is plane, and whose posterior surface has the same curvature as the anterior surface of the object lens. The interposition of this meniscus necessarily increases the distance at which the image of the object is formed behind the object lens; so that if we keep the object-glass micrometer at the same anterior distance, we must lengthen the body of the microscope in order to see clearly the image in the diaphragm, or what leads to the same result, we may leave the body of the microscope unaltered, and increase the distance of the object from the lens. The amount of this alteration depends upon the curvature of the object-lens and upon the nature of the substance interposed. If the curvature is known, we deduce from this amount, the ratio of refraction of the substance. If it is unknown, and it is always best to make this supposition, then we begin by forming the meniscus with pure water, whose ratio of refraction is known, being, according to Newton, equal to or 1,33586; and by comparing the alteration of distance necessary in this case, with that required for the particular substance afterwards subjected to experiment, we deduce the ratio of refraction of this substance compared with that of water. The application of this process requires much skill and address, because the accuracy of it depends upon the more or less perfect exactness with which we bring the image into the diaphragm, in doing which, our only guide is the condition of greatest distinctness of vision.

Still it admits of many very useful applications, both for substances which only become transparent when reduced to their laminæ, and for liquids of which we possess only very small quantities, a single drop being sufficient for the observation.

Amplifying Glass and Achromatic Eye-Glasses.

199. THE imperfections of the microscope arise in a great degree from the want of achromatic lenses, which is the more felt according as we attempt to make use of higher degrees of magnifing power. Unfortunately it is impossible to remedy this defect entirely, since it is in vain to think of forming achromatic lenses so small as those which the microscope requires. But this evil may be, to a considerable degree, removed by a method which suggested itself to practical men, before any theory had been devised to explain its effect, or to show how it might be employed in the most advantageous manner. This method consists in placing, in the interior of the microscope, behind or before the first image f,,, a third converging glass, of a focus properly determined. Then the course of the rays will be such as is represented in figures 123, 124. The first disposition, figure 123, was invented by Campani; the other, figure 124, by Ramsden.

The employment of this glass, called the amplifying glass, is common to all dioptric instruments. Its evident use is so to collect the pencils separated by the object-glass, to concentrate them in a smaller space, and thus to render the image more distinct, smaller, and consequently to make a greater part of the object visible by a given eye-glass. But there is another less obvious use, which consists in the achromatic influence which this glass exerts. 200. When the rays coming from an object, have been refracted by any system whatever of spherical lenses, which form images of it about the axis AX, owing to the unequal refrangi- Fig. 125. ·bility of light, the foci of the rays of different colours are not, in general, formed at the same distance; so that if ø, is the focus of the violet rays, 9, will be that of the indigo, p, that of the blue, and finally, that of the red; and as the same property

2

3

Fig. 123.

2

belongs to the radiating points situated without the axis, there
will be formed in 1, a violet image VV, in 9, a blue image BB,
and in 97
a red image RR; and the same cause which distri-
butes them at different distances will give them also different
dimensions. Now, if the eye be placed at some point O, in the
axis AX, for the purpose of looking at these images, it will, in
the first place, suffer the inconvenience of their unequal distance,
which will prevent them from being seen together at the exact
distance necessary for distinct vision. Moreover, it will be dis-
agreebly affected by the inequality in their size; for, as they
project beyond one another, they will exhibit the outlines of
objects bordered with coloured fringes of red, violet, or the
intermediate colours, according as one or the other may predom-
inate, in the course of the refractions they severally undergo. A
great advantage, therefore, will be obtained if we can regulate
the size of these images, in such a manner as to make them ex-
actly proportional to their distances from the eye, as represented
in figure 126; for then the eye seeing all their borders in the
same straight line VRO, will receive at once, from these borders,
the sensation of all these species of rays, and consequently the
coloured fringes will disappear. Now this disposition, which
would seem at first to be very complicated and difficult, is found
in fact to be extremely simple; and is precisely the effect pro-
duced by the amplifying glass, when its focal distance, and its
position with respect to the other glasses, are properly determin-
ed. This can be done only by calculation, and consequently, I
cannot here state the conditions. I will simply remark, that this
arrangement, in order to be possible, requires that there should
be in the instrument, at least two glasses A, A,, besides the
object-glass A,; for, with the object-glass and only one other,
we should not be able to render the borders of the last image
colourless, except in one particular situation of the object. Ac-
cordingly, in all the applications that follow, the achromatic eye-
glass must always be composed of at least two lenses.

201. The achromatic eye-glass of Campani, is the one always employed in compound microscopes, and generally, in instruments where we do not wish to extend fixed wires over the image given by the object-glass. But when these wires become necessary, as in astronomical instruments destined for measuring, where we are obliged to fix precisely the direction of

the rays which come from the heavenly body to the eye, at a known instant, we cannot employ this arrangement; because, in drawing out or pushing in the eye-glass to accommodate it to different eyes, we should necessarily move the wires, and if this movement were not exactly in the axis of the telescope (and we cannot suppose it would be), the successive passages of the body would not be comparable with each other. In this case, the eye-glass of Ramsden is particularly applicable; because, Fig. 124.. being situated completely beyond the first image f, 9, it can be drawn out or pushed in, without moving the wires extended at the place where the image is formed. Accordingly, we always employ it under these circumstances, and it was for this purpose that that celebrated artist contrived it. By examining the effect of the eye-glass of Campani, in microscopes, M. Cauchoix found it advantageous to give to the amplifying glass the form of a meniscus convex towards the object-glass. As to the enlargement produced by this apparatus, we shall explain hereafter the means of determining it according to a process devised by M. Arago, and which is applicable to all optical instruments. We repeat this observation for two different distances of the object from the object-glass; which will cause the image to advance or recede, and make it necessary to move the two lenses of the compound eye-glass, in order to bring it to the true point for distinct vision. We shall thus have two known enlargements, for a known lengthening of the tube; their difference, distributed uniformly among all the intermediate lengthenings, will give the corresponding intermediate enlargements, which may be traced on the tube.

A bare inspection of figures 123, 124, will show that these eye-glasses do not change the inversion of the objects produced by the object-glass; but their erect position may be restored by employing more than two glasses, as we shall see hereafter. This is done, in certain cases, in the telescope, which is next to be considered.