Opticians have long since been able to effect the required corrections, with sufficient accu racy for most practical purposes, in the con struction of large object glasses for telescopes ; the size of which has been only limited by the impossibility of obtaining glasses of large di mensions perfectly free from faults. But it has been only of late years, that the construc tion of achromatic and aplanatic object-glasses for microscropes has been considered prac ticable,—their extremely minute size appearing to forbid the employment of the necessary combinations, since a very high amount of accuracy is required in the several curvatures, in order to obtain any real improvement. About the year 1820, however, the attempt was first made in France by M. Selligues, who was fol lowed by Frauenhofer at Munich, by Amici at Modena, and by Mr. Tulley of London ; and with these attempts a new era in the history of the microscope may be said with truth to have commenced. The work has been prosecuted, both theoretically and practically, with the greatest zeal, and the result has been most successful. By combining two or three groups of double lenses, each corrected in a particular manner, so that the whole is quite free from aberration, a perfectly sharp and clearly defined image may now be obtained through a lens of many times the aperture of those formerly in use ; and the differences in the representation of the objects under enquiry, between such lenses and a good achromatic, are such as could not have been, it priori, sus pected. One of the most pleasing results of this improvement has been the greatly-increased unanimity amongst microscopical observers, as to the appearances actually witnessed by them; for with the old and imperfect instru ments, great uncertainty could not but exist in regard to many objects, of whose nature every one formed his own opinion, frequently accord ing to preconceived ideas ; but at present the objects are presented to the sight of each ob server possessed of a good instrument, with so much more clear and uniform an appearance, that there is much less scope for the play of his imagination as to their real character,— however much he may exercise it upon their history. It would be foreign to the purpose of this article to enter into scientific details upon the minutiae of the construction of achro matic combinations; but it may not be amiss to state that, in the opinion of the author, English artists have far surpassed foreigners in the construction of lenses of very short focus, whilst some foreign combinations which he has seen, of low magnifying power, possess an advantage over those of British make,—the constructors of the latter having sometimes sacrificed what he deems adequate correctness in aiming at a very large aperture." With these preliminary details as to the na ture of the means by which microscopic power is obtained, we shall proceed to notice their chief applications to practice. Excluding for the present the solar and gas microscopes, in which an image visible to any number of per sons at once is formed upon a screen, and is viewed by them precisely as other surrounding objects would be, we shall consider the instru ments (to which the term microscope is more commonly applied), whose effects are produced by their influence on the rays of light which enter the eye of the observer, and which can be used, therefore, by but one at the same time. These are distinguished as single or simple, and compound microscopes. Each of these kinds has its peculiar advantages for the anatomist; and we shall, therefore, describe the construction and uses of both in some detail. Their essential difference consists in this,—that in the former the rays of light which enter the eye of the observer proceed directly from the object itself, after having been subject only to a change in their course, whilst in the latter an inverted image of the object is formed by a lens, which image is viewed by the ob server through a simple microscope, as if it were the object Itself. The simple microscope may consist of one lens, but (as will be pre sently shown) it may be formed of two or even three; but these are so disposed as to produce an action upon the rays of light correspondent to that of a single lens. For this kind of mi croscope, therefore, we prefer the term simple to single. In the compound microscope, on the other hand, not less than two lenses must be employed, one to form the inverted image of the object, and this being nearest to it is called the object-glass, whilst the other mag nifies that image, being interposed between it and the eye of the observer, and is hence called the eye-glass: Both these may be constructed of several lenses, as will be hereafter shown ; but they are so arranged as to have the func tions of a single lens, and are only combined for the purpose of correcting the defects inci dental to it.
In order to gain a clear notion of the mode in which a single lens serves to magnify minute objects, it is necessary to revert to the pheno mena of ordinary vision. An eye free from any defect has a considerable power of ad justing itself in such a manner as to gain a distinct view of objects placed at extremely varying distances; but the image formed upon the retina will of course vary in size with the distance of the object ; and the amount of detail perceptible in it will follow the same proportion. To ordinary eyes, however, there is a limit within which no distinct image can be formed, on account of the too great diver gence of the rays of the different pencils which then enter the eye; since the eye is usually adapted to receive and bring to a focus rays which are parallel or slightly divergent. This
limit is variously stated at from five to ten inches ; we are inclined to think from our own observations, that the latter estimate is nearest the truth ; that is, although a person with ordi nary vision may see an object much nearer to his eye, he will see little if any more of its details, since what is gained in size will be lost in distinctness. Now the utility of a con vex lens interposed between a near object and the eye consists in its reducing the divergence of the rays forming the several pencils which issue from it; so that they enter the eye in a state of moderate divergence, as if they had issued from an object beyond the nearest limit of distinct vision ; and a well-defined picture ie consequently formed upon the retina. But not only is the course of the several rays in each pencil altered as regards the rest by this refracting process, but the course of the pencils themselves is changed, so that they enter the eye under an angle correspondent with that at which they would have arrived from a much larger object situated at a greater distance. The picture formed upon the retina, therefore, corresponds in all respects with one which would have been made by the same object, greatly increased in its dimensions, and viewed at the smallest ordinary distance of distinct vision. A short-sighted person, however, who can see objects distinctly at a distance of three or four inches, has the same power in his eye alone, by reason of its greater convexity, as that which the person of ordinary vision gains by the assistance of a convex lens which shall enable him to see at the same distance with equal distinctness. It is evident, therefore, that the magnifying power of a single lens, depending as it does upon the proportion between the distance at which it renders the object visible, and the nearest distance of unaided distinct vision, must be different to different eyes. It is ordinarily estimated, however, by finding how many times the focal length of the lens is contained in ten inches ; since, in order to render the rays from the object nearly parallel, it must be placed very nearly in the focus of the lens; and the picture is referred by the mind to an object at ten inches distance. Thus, if the focal length of a lens be one inch, its magnifying power for each dimension will be ten times, and consequently a hundred super ficial; if its focal distance he only one-tenth of an inch, its magnifying power will be a hundred linear or ten thousand superficial. The use of the convex lens has the further ad vantage of bringing to the eye a much greater amount of light than would have entered the pupil from the enlarged object at the ordinary distance, provided its own diameter be greater than that of the pupil. It is obviously neces sary, especially when lenses of very high mag nifying power are being employed, that their aperture should be as large as possible; since the light issuing from a minute object has then to be diffused over a large picture, and will be proportionally diminished in intensity. But the shorter the focus the less must be the dia meter of the sphere of which the lens formi a part; and unless the aperture be proportionally diminished, the spherical and chromatic aber rations will interfere so much with the distinct ness of the picture, that the advantages which might be anticipated _from the use of such lenses will be almost negatived. Nevertheless, the simple microscope has always been an in strument of extreme value in anatomical re search, owing to its freedom from those errors to which, as will hereafter appear, the com pound microscope is subject; and the greater certainty of its indications is evident at once from the fact, that the eye of the observer receives the rays sent forth by the object itself, instead of those which proceed from an image of that object. A detail of the means em ployed by different individuals, for procuring lenses of extremely short focus, though pos sessing much interest in itself, would be mis placed here ; since recent improvements, as will presently be shown, have superseded the necessity of all these. It may, however, be stated that Leeuwenhoeck, De la Torre, and others among the older microscopists, made great use of small globules procured by fusion of threads or particles of glass. The most im portant suggestion for the improvement of the simple microscope composed of a single lens proceeded some years ago from Dr. Brewster, who proposed to substitute diamond, sapphire, garnet, and other precious stones of high re fractive power, for glass, as the material of single lenses. A lens of much longer radius of curvature might thus be employed to gain an equal magnifying power, and the aperture would admit of great extension without a pro portional increase in the spherical and chro matic aberrations. This suggestion has been carried into practice with complete success as regards the performance of lenses executed on this plan ; but the difficulties of various kinds in the way of their execution are such as to render them very expensive ; and as they are not superior to the combination now to be de scribed, they have latterly been quite super seded by it.