326 THE SCIENCE OF MICROSCOPY. 



sight merely. For many problems of micro-physics whose 

 starting point may be the object itself, must be solved by 

 deduction from the nature of the diffraction phenomena observed^ 

 and the resolving and defining functions upon which the quality 

 and character of the image depends. The theory of the micro- 

 scope, and the action of the illuminating apparatus employed with 

 it, enter largely therefore into our reasonings, and it is plain that 

 practical microscopy must be pre-eminently a science of 

 observation, though its immediate interests and aims are not 

 those of pure optical science, while they also differ from the 

 purely technical purposes and applications of the constructor. It 

 is, moreover, plain that in drawing inferences respecting the 

 constitution of objects which can only be studied through the 

 medium of their enlarged images, our recourse to physical and 

 physiological law must be more frequent and more exact in 

 application than is needed where we have simply to grasp what we 

 see under ordinary circumstances. 



Yet in general practice ^*^ familiarity with the microscope" does 

 not mean acquaintance with its mode of action, but simply 

 readiness in handling it. And '^management of illumination" 

 does not mean putting theory in practice, but the production of 

 certain effects. The microscopist accepting the image before him 

 as conveying " the truth and nothing but the truth," does not 

 study these effects of illumination as optical phenomena, but as 

 objective material facts. Raising no question of identity or 

 conformity of the image with the object, he proceeds to build up 

 evidence of fact for use in the investigation on which he is intent, 

 without sifting the evidence upon which his facts may be 

 warranted to be worthy of trust. Hence the conflicting observations 

 and descriptions by different observers even where the same 

 object is under examination : hence, also, the false doctrines built 

 upon false facts, and erroneous interpretations. 



This notorious discrepancy of results (which if beyond correction 

 would at once condemn microscopy as an exact science) is com- 



