336 



MICROSCOPE. 



withoul any essential difficulty in practice; for 

 the dispersive power of flint-glass is so much 

 greater than that of crown-glass, that a convex 

 lens of the former, the focal length of which 

 is 7jj inches, will produce the same degree of 

 colour with a convex lens of crown-glass whose 

 focal length is 4 inches. Hence a concave 

 lens of the former material and curvature will 

 fully correct the dispersion of a convex lens of 

 the latter, and will yet diminish its refractive 

 powvr only to such an extent as to make its 

 focus ten inches. The correction for chromatic 

 aberration in such a lens would be perfect, if 

 it were not that, although the extreme rays, 

 violet and red, are thus brought to the same 

 focus, the dispersion of the rest is not equally 

 compensated ; so that what is termed a secon- 

 dary spectrum is produced, the images of ob- 

 jects seen through such a lens being bordered 

 on one side with a purple fringe, and on the 

 other with a green fringe. Moreover such a 

 lens is not corrected for spherical aberration ; 

 and it must of course be rendered free from 

 this, to be of any service, however complete 

 may be its freedom from colour. 



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, a 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 

 mat/ 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 



* Those who wish to study the principles which 

 now guide opticians in their construction, should 

 refer to Mr. J. J. Lister's paper in the Phil. Trans, 

 for 1829, and Mr. Ross's Memoir in the Trans, of 

 the Society of Arts, vol. ii. 



