THE SCIENCE OF MICROSCOPY. 3 19 



developed in the image. All detail that appears in the image is 

 just what, and no more than what, is delineated by those pencils 

 of light whose points fall on the conjugate focal plane of the 

 objective within the microscope. The intervention between 

 object and image in many distinct series of refractions, constitu- 

 ting in sum the image-forming process of the microscope, demand 

 the same discriminating study of effects on the part of the 

 microscopist, who has to draw inferences respecting the nature of 

 the object from the manner in which it is represented in its image, 

 as it does on the part of the optician, who has to test the correct- 

 ness of his lens construction by analysis of its performance. But 

 since the comparatively simple test-procedure of the optician is 

 conducted with the help of Jcnown objects, while the microscopist 

 has to study a variety of unhioivn structures (mostly of a more 

 complex kind), the preparatory studies of the latter must be wider 

 and more varied in accordance with the complexity of the objects, 

 and the greater difficulty of interpreting their shadow images. 

 The technical points of interest in the analysis of an image 

 which occupy the optician, e.g., correct geometric delineation, 

 achromasy, and light intensity, are also of cardinal import- 

 ance to the practical microscopist. But he will not find 

 much advantage in setting up confused schemes of 

 ''penetrating" and ''resolving'' poivers to aid his inter- 

 pretation of the image (theoretical hypothesis introduced 

 be it observed — by microscopists who claim to be eminently 

 practical*). He will probably gain a clearer insight into the optical 



* The great philosopher, from whose disciplined reasoning as well as 

 observing faculties emanated the suggestion of the defining and penetrating 

 power of the telescope, is not answerable for its transference to a theory of the 

 microscope. The space-penetrating telescopic power — a function of angular 

 aperture in collecting feeble light from distant objects — is so glaringly 

 opposed to the conditions obtaining in the microscope (nearness of object and 

 command of intense illumination independent of objective aperture), that it 

 could not possibly have been made the basis of explanation of a microscopic 

 " penetrating power " if the microscopists of the day had rightly understood 



