212 ON THE THEORY OF THE MICROSCOPE. 



stituent element of a compound system of lenses in tlie joint 

 performance of the whole. The absence of any sure guide — in 

 other words, the want of such adequate conception of the optical 

 fnnctions of objective and ocular respectively, as might bring into 

 instructive contrast whatever essential differences exist between 

 the functions of these two portions of the microscope, whilst 

 at the same time that which is merely incidental and 

 unimportant is eliminated — has been, and stiil is, the cause 

 of serious defects which have hitherto attached to theories of 

 the microscope, and also the occasion of certain mis-directions 

 which have been taken in the endeavour to render it more 

 perfect. When then we define the function of the objective to 

 be, the production of a real image, and the function of the eye- 

 piece, the amplification of this image, — such explanation, 

 however true and serviceable to a certain degree, does not by any 

 means reach the essential principle of action of the compound 

 microscope. This is obvious at once when we consider that by 

 such a definition the combination of objective and eye-piece is 

 made only to indicate magnifying power, whereas on the contrary 

 the remarkable superiority of compound over simple microscope 

 consists in the quality of Us performance^ even with such moderate 

 magnifying power as the simple microscope can reach without 

 much difficulty. On the other hand, the proper signification of 

 the principle of combination is indicated in this, that a character- 

 istic division of work obtains in respect to the focussing function 

 by which an image is formed, and the function by which 

 that image is spread out over a much larger area, these functions 

 being carried on in such wise that the specific modus operandi of 

 the first belongs to the objective, that of the latter to the 

 eye-piece. By the objective an image ^ is formed and spread out in 

 what is practically an almost perfect accordance with the laws by 

 which images of infinitely small elements of a surface are formed. 

 Ey the eye-piece a displacement of focus is effected; that is to 

 eay, a change of divergence of each separate pencil of light takes 

 place till the divergence is almost imperceptible, and the pencils 



