310 THE SCIENCE OF MICROSCOPY. 



second heading includes, besides the individual qualifications of the 

 observer, a critical review of actual and possible causes of a variety 

 of microscope effects observed in practice, which can only be 

 explained by reference to the peculiar conditions of microscopic 

 vision. So far as the technical estimate of construction and 

 performance of the microscope is concerned the practical optician 

 is held chiefly accountable, the microscopist relying simply on the 

 warrant of the maker or, occasionally, on the authority of some 

 expert. It is curious, however, to note that a practice has sprung 

 up amongst microscopists — and has become the favourite occupation 

 of amateurs who ignore the theory of the microscope — of applying 

 certain "tests" of its performance, which consist in obtaining 

 certain appearances recognised and interpreted as a resolution of 

 the "test object " used, without any attempt to determine whether 

 these appearances are optical characteristics of an image formed 

 under given conditions, or ph7sical realities of the object itself. 

 Whereas the true rationale of such test-procedures carries the 

 scientific observer back to the analysis of the image-forming 

 functions of microscope lenses, and the physiology of vision 

 modified by the unusual conditions under which a microscope 

 image is seen. The result of optical analysis is briefly this — that 

 theory and practice (as hitherto conducted) stand in direct 

 contradiction ! So much the worse for — which ? 



Unhappily the word '' theory " is too frequently a stumbling 

 block, and the interest felt by '' theorists " and ^' practical micro- 

 scopists " is commonly represented as distinct in kind and aim, the 

 consideration of the microscope as an object of physico-mathe- 

 matical research being allotted to the former, while some more 

 tangible result is expected from the latter. But setting this view 

 aside as neither correct nor just, it may rather be urged that 

 *' theory " means, in the case of the microscope, a demonstrated 

 theorem, and therefore that its conclusions must be accepted as 

 ruling principles to be applied by the practical microscopist. In 

 doing this he need not set about re- discovering for himself 



