294 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ledge of nature. " There remains, of course," says the author, " the 

 consolation that there is much between heaven and earth that is not 

 dreamt of in our philosophy. Perhaps in the future human genius may 

 succeed in making forces and processes serviceable which may enable 

 the boundaries to be overstepped which now seem impassable. Such 

 is, indeed, my idea. I believe, however, that those instruments 

 which may perhaps in the future more effectively aid our senses in 

 the investigation of the ultimate elements of the material world than 

 the Microscope of the present, will have little else than the name in 

 common with it." 



There is, therefore, but small scope left for the advance of optical 

 art, in regard to the most important point in the efficiency of the 

 Microscope — aperture — the possible direction of which has been indi- 

 cated in the foregoing observations. The further perfecting of the 

 instrument must chiefly relate to the two other factors of its optical 

 performance, viz. the magnifying power and the dioptrical exactitude 

 with which the image is formed. In these is to be found the most 

 important task left for the optician in reference to the Microscope. 



With regard to the first — the amplification of the image — the 

 author proceeds to explain in outline the point since dealt with more 

 in detail in his papers on the relation of aperture to power,* and 

 the uselessness of a magnifying power that is out of proportion to the 

 aperture — increased size without visible detail, so that we have 

 " mere emptiness." There is room for improvement of the eye-piece in 

 reference to many points — the size of the field, the uniformity of the 

 magnifying power, &c. ; but they are points of subordinate importance, 

 because they do not touch the performance of the Microscope in its 

 most essential respects. The practical optician has, it appears, 

 adopted this view. At least the fruitless efforts to increase the actual 

 capacity of the Microscope by special eye-pieces have ceased. 



It is an essentially different matter with the remaining factor. 

 The conditions on which depend the more or less perfect union of the 

 rays in an optical system are so manifold and so complex, and the 

 ways and means of satisfying certain requirements are so numerous 

 that a wide field will remain open to optical science for all time. The 

 imperfection of the optical image at the focal point springs from two 

 causes very similar in their effects. The one arises from the residual 

 spherical and chromatic aberration which even the best devised com- 

 binations of refracting media still leave ; the other lies in the want of 

 homogeneity, precise form, and exact centering of the lenses which 

 even the most perfect art can never wholly remove. The result is that 

 every objective unites the cones of rays proceeding from the points 

 of the object, not in mathematically exact image points, but in light 

 surfaces of greater or less extent — circles of dissipation — and thereby 

 limits the distinct representation when the details are of a certain 

 minuteness. 



Of course every part of the optical system, the objective as well as 

 the eye-piece, contributes to this imperfection of the image. In its 

 practical importance, however, the part played by each of the elements 



* See this Journal, ii. (1882) pp. 300 and 460, and iii. (1883) p. 790. 



