By an achromatic object-glass for a com pound microscope, therefore, is not meant one which simply contains within itself a perfect correction for its own errors, but one in which the usual order of dispersion is so far reversed that the light, after undergoing the series of changes effected by the eye-piece, shall come uncoloured to the eye. " We can give no spe cific rules," says the writer of the article just referred to, whom we believe to have had great practical experience in the matter, " for pro ducing these results. Close study of the for mula: for achromatism given by celebrated ma thematicians will do much ; but the principles must be brought to the test of repeated expe riment. Nor will the experiments be worth any thing, unless the curves be most accu rately measured and worked, and the lenses centred and adjusted with a degree of preci sion which, to those who are familiar only with telescopes, will be quite unprecedented." We are not favourable to the use of a very high magnifying power in the eye-piece; and we believe that it will be discontinued in propor tion to the perfection attained in object-glasses. We have seen microscopes of foreign construc tion in which only object-glasses of compara tively long focus were employed, and the re quired power was made up by the great con vexity in the eye-glass; but the performance of these was not to be compared to that of instru ments of British manufacture. Our own expe rience leads us to think that there are very few objects of which more can be made out with a deep than with a shallow eye-piece,—the dimi nution in distinctness and loss of sight being nearly sufficient to counterbalance the gain derived from increased power. Hence the mag nifying power of an instrument is by no means to be regarded as an indication of its excel lence; for that is to be considered as the best which, eeteris paribus, will show the most with the lowest power. It may be scarcely an exaggeration to affirm that there are few objects of which the details may not be as well made out by an achromatic microscope magnifying but 100 diameters, as by the best ordinary microscope magnifying 1000 ; and there are many objects shown with the greatest readi ness by the former, which are totally inscru table by the latter.
Next in order of optical perfection to the achromatic microscope with the Huyghenian eye-piece, we are disposed to rank the doublet microscope, invented by Mr. Holland. This gentleman has proposed to adapt to his dou blets and triplets a compound body, con taining an eye-piece somewhat resembling the Huyghenian, but differing from it in having the lenses fixed at a distance equal to the whole sum of their foci. " By this increase of dis tance, light and defining power are gained, although the magnifying power and the field of view are diminished ; but at the same time the latter is rendered very perfect." Having ourselves had a microscope constructed upon this principle, we can speak in very high terms of its performance. The field of view will appear very small to those accustomed to the use of eye-pieces of high power; but every part of it is brilliantly illuminated, and diffi cult test-objects are exhibited by it with a sharpness and definition which we have seldom seen equalled. For objects which require to have a large surface in view at once, such a microscope is inappropriate ; but for the pur pose of minute examination of those in which the parts may be studied independently, we regard it as the best substitute for the achro matic microscope; and we can strongly re commend it to those who are debarred by the price of the latter instrument from possessing themselves of it. In employing doublets or
triplets of high power as objectives in such a microscope, care must of course be taken (as when they are used singly) to avoid injuring them by contact with the object. For the most difficult test-objects, a triplet of ith of an inch focus should be employed ; and the powers should successively diminish down to a dou blet of ith of an inch, which may be advanta geously employed for opaque objects. It is necessary to state that the performance of this microscope will very much depend upon the attention paid to the illumination of the object; on this we shall hereafter treat in detail.
We shall next speak of eye-pieces which are intended to increase the size of the field in the most advantageous manner, without regard to the perfection of minute details. This ob ject is ordinarily attempted by the substitution of two plano-convex lenses, or double-convex lenses of low curvature, for the single lens of the eye-glass. It has been proposed to make the same change in the field-glass ; but, in our opinion, the loss by reflexion from two addi tional surfaces is by no means compensated by the diminution of aberration. We are our selves, however, in the habit of employing an eye-piece, which we regard as greatly superior to that in ordinary use. It consists of a me niscus having the concave side next the eye, and a convex lens having the form of least aberration, with its flattest side next the ob ject, nearly resembling, therefore, Herschel's aplanatic doublet. The field-glass is a double convex lens of the form of least aberration. With this eye-piece we are enabled to obtain a field of 14 inches diameter (measured at the usual distance-10 inches) equally distinct and well illuminated over every part, and admi rably adapted for the display of sections of wood, wings of insects, and objects of a simi lar description, and also for opaque objects. When employing it for these purposes, we much prefer the use of ordinary double-convex objectives to achromatic lenses ; for the latter, being adjusted for a much smaller field, pro duce an image which is only distinct in the centre; and the former, being of low power, may have an aperture quite sufficient to admit the requisite amount of light. Even with deeper objectives, the performance of this eye piece approaches much more closely to the effect of an achromatic microscope, than would be supposed by those who have only seen the ordinary one; and, when made on a small scale, it may be advantageously substituted for Mr. Holland's for all but the most difficult objects. The two additional surfaces are of course disadvantageous, by reflecting some of the fainter rays proceeding from the more delicate markings; but the increased magnify ing power is gained with so little aberration, that a considerable general advantage is hence derived. We can regard no microscope as complete, without an eye-piece of this kind, with a set of ordinary objectives of low powers; for it will certainly do what no other combi nation with which we are acquainted can effect. The great improvements recently made in the construction of achromatic objectives, and the unquestionable fact that, for exhibiting the minute details of objects, they are infinitely superior to all other kinds, have had, we think, a tendency to blind microscopists to the ad vantages afforded by other combinations, where it is desired to obtain a view of the general arrangement of the parts of a large object, rather than to investigate its minutia:.