320 THE SCIENCE OF MICROSCOPY. 



characteristics of the image which he has to interpret, if he study 

 the several phases of its actual formation by the lenses of the 

 microscope. When he fairly realises the one grand function, 

 namely, a perfect re-union in the final focal plane of delineating 

 pencils of light' collected first by the objective, and then again 

 spread out and re-collected, so that their points shall fall in the 

 same exact order and relation of space on the enlarged area which 

 they originally had in the illuminated object, he will better 

 comprehend the several causes of imperfection which attend so 

 complicated a focussing function in its geometric projection of an 

 image : and will also perceive that the so called "' powers '' of the 

 microscope are but mythical expressions, indicating the relative 

 success with which certain optical disabilities of our present lens- 

 material have been removed or '^ compensated.'' He will further 

 know how be^t to connect in his own mind the visible detail of the 

 image with the absolute or presumptive constitution of the objects 

 and also to conjecture the causes which render actually existing 

 . detail non-apparent. A theory of the microscope thus simply 

 expressing the optical phases undergone by pencils of light in their 

 passage from the object until they reach the eye, exhibits all the 

 functions of the microscope in their natural inter-dependence 

 without any break, excepting when any new phase, initiated by the 

 intervention of refracting media, leads to some special point for 

 consideration. And such a theory forms the only safe foundation 

 for a real science of microscopy. Considered from this point of 

 view the optical conditions which obtain outside the microscope, 

 but which rule the action within to such an extent that the most 

 perfect image can only be realised by including them in the general 

 problem, are scarcely second in importance to the action of the 



the distinction. There are other explanations of "penetrating power" 

 current at the present day which are as little warranted by the optical 

 principles. How long will the microscopist rest content with being the only 

 scientist whose knowledge of his instrument is unworthy of it, and whose 

 mode of using it may often be placed on a parallel with that of the exhibition 

 of a peep-show. 



