MICROSCOPIC VISION. 165 



Therefore for objectives of 0*9 N.A. and upwards it is neces- 

 sary to revert to the 70,000 times the N.A. limit. 



We now come to a very important point ; viz., the " black 

 and white dot." This term is used to express the fact that 

 when an object — such, for example, as a siliceous plate — is 

 viewed under the microscope, its edge assumes either a 

 black or white appearance, according to changes in focus ; 

 but when we have the edge an inner edge of a hole, then 

 when the hole is very minute the black edge on one side of 

 the hole will meet the black edge on the opposite side, and 

 the hole will appear as a " black dot " ; but when the focus 

 is arranged so as to give a white edge, then the hole will, by 

 a similar argument, become a " white dot." 



This is a phenomenon perfectly familiar to diatomists, 

 who are accustomed to study the minute holes in the 

 siliceous valves of diatoms ; hence the origin of the some- 

 what curious name given to this phenomenon of " black and 

 white dot." 



Although this term was primarily applied to diatoms, it is 

 applicable to all minute microscopical objects, such as bac- 

 teria, hairs, flagella, and the edges of objects generally. 



Now there is a curious point about this phenomenon, 

 which bears on the subject under discussion this evening ; 

 viz., that the visibility of the " black and white dot " 

 depends upon the aperture of the objective, because the 

 greater the aperture the easier it is to obtain a " black dot." 

 When, however, the hole becomes excessively minute, a 

 ^' black dot " is no longer attainable, and we have to content 

 ourselves with the " white dot " appearance. 



There is nothing in the theory of microscopic vision, as at 

 present enunciated, to account for this fact, that a larger 

 aperture is required to resolve the " black " than the ^' white 

 dot " image. 



