We should recommend, therefore, that every achromatic microscope should be fitted with at least two Huyghenian eye-pieces, adapted ex pressly to the achromatic objectives; and that it should also have a meniscus eye-piece, with a set of ordinary object-glasses of long focus. And the best substitute for such a microscope, at least for the purposes of anatomical or phy siological research, we believe will be found in Mr. Holland's doublet microscope, which should be furnished with his eye-piece for doublets already described, and with a me niscus eye-piece and ordinary object-glasses of low power. In this last form of compound microscope, there is the further advantage, that the high magnifying power of the doublets and triplets employed as objectives renders them available as simple microscopes; and this cannot be said of achromatic object-glasses, which have not yet been usually made of shorter focus than one-tenth of an inch, and which are not, therefore, of much use in them selves. No one, however, can be regarded as entitled to form positive conclusions in regard to difficult questions of microscopic enquiry, until he has availed himself of the very best means of observation at his command ; and these are certainly to be found only in achro matic microscopes of the highest class.
For viewing large opaque objects, achromatic objectives of low power are often very useful, on account of the large quantity of light they admit, which supersedes the necessity of arti ficial illumination; this is a particular advan tage in anatomical investigations, in which it is often especially necessary to avoid the re flection of condensed light from the surface of the object, on account of the confusion which is thereby occasioned.
The achromatic objectives at present usually made on the continent consist of sets of three or more, of which one, two, or three may be used at once. In this manner considerable variety of power may be gained ; but the highest degree of perfection in the performance must be sacrificed to obtain it, since no single objective consisting of two lenses only can be thoroughly corrected, and each combination ought to be corrected for itself alone. The best achromatics made by British artists consist of combinations of two or three compound lenses, which cannot be separated ; and thus every required power must be furnished by a distinct combination. The expense of a mi croscope fitted with the requisite number of these, however, is a great bar to its general employment. Other combinations have been constructed, therefore, in which the lens next the object may be removed, so as to diminish the magnifying power considerably ; and the corrections are so adjusted as to be nearly the same when the two or when three compound lenses are used together. The difference be
tween the performance of the best of these, and that of those most perfectly adjusted, is not, for general purposes, of much importance. Two sets of these separating lenses,—a high and a low one,—giving four powers, therefore, which may range from an inch and a half to one-eighth of an inch focus, will adapt the microscope, with the eye-pieces we have men tioned, to a great variety of purposes.
The power may be further varied by length ening the body of the microscope, by drawing out the eye-piece, which should always he made capable of this kind of movement. This operates by increasing the distance from the object-glass of the image formed by it, and therefore augmenting the site of the image; the object must of course be brought some what nearer on the other side. We have al ready stated that the length of the body cannot be much increased with advantage; but a mo derate variation will be found useful in many ways. It enables the magnifying power to be adjusted to almost any point intermediate between those given by the different objectives. Thus, one may give a power of 80 diameters, and another a power of 120 ; by using the first, and drawing out the eye-piece, the power may be increased to 100. Again, it is often very useful to make the object fill up the whole, or nearly the whole, of the field of view. This is especially the case, when it is itself not very transparent, and requires a strong light to render its details visible ; in which condition a glare entering around its edges would very much interfere with its distinctness. When opaque objects, also, are being viewed by con densed light, in the modes hereafter to be stated, it is often extremely desirable to make them, or the discs on which they are mounted, fill up the whole field. In either case the drawing out of the eye-piece until the end is accomplished answers the object most simply and effectually. In the use of the micro metric eye-piece, also, which will he presently described, the capability of adjusting the mag nifying power to a certain definite amount will be found of very great utility. It is to be borne in mind, however, that for giving the highest effect to the achromatic objectives, a certain fixed distance of the eye-piece is neces sary; this is usually adjusted by the maker; but it may be easily determined by trial, since at any other the want of correction of the chro matic aberration will make itself apparent (however slightly) by the presence of coloured fringes around the images.