LIiriTS OF OPTICAL CAPACITY OF THE MICROSCOPE. 411 



One may be well excused from referring to the meagre optical 

 chapters in our handbooks on the microscope, which might perhaps 

 suit the '*Boys'-own-book," but which contain neither demonstration 

 nor diagram of the course of rays through any sort of modern lens 

 system, nor even a rough application of its very elementary 

 statements respecting refraction and reflection to any special formulae 

 of constructions, according to which the lens combination of an 

 objective would be worked, or by which its performance would be 

 tested. l^OT can the favorite descriptive chapter of the instruments 

 of various makers help anyone to a theory of the microscope. The 

 opinions expressed by experts and authorities on definition, 

 penetration, resolution, aperture, etc., as being so many separate 

 powers or qualities, besides savouring strongly of a mythological period 

 in the history of the microscope, have only retarded the search in 

 the right direction, viz., by physical analysis and physiological 

 study of optical phenomena for true causes of the effects observed. 

 And in fine it must be confessed that our handbooks fail greatly in 

 respect to theories of the microscope, however valuable their 

 information on practical and mechanical subjects, and more 

 especially on all branches of science involving skilful use of the 

 instrument. 



In the absence of such handbooks as the German students possess, 

 and of which the work of Nageli and Schwendener might be cited 

 with admiration as an example, the scattered articles and shorter 

 notices in our serials rise into comparative importance. But it will 

 scarcely be contended that such desultory and disconnected com- 

 munications and such remarkable disputes respecting easily deter- 

 mined facts, should be accepted as an equivalent of the systematic 

 theory and practical demonstration which distinguish foreign study 

 of optics applied to the microscope, from our yet unlearnt, or at 

 least unwritten, micrograph ic science. 



Various communications bearing more or less on the optical 

 capacity of lens- systems constructed on given formulae or for 

 employment as "dry" or ''immersion" objectives, have appeared 

 in the Monthly Microscopical Journal, the Quarterly Journal of 



