246 ON THE XnEORY OF THE MICROSCOPE. 



being equally distinct in origin and character, and also 

 capable of being separated and examined apart from each 

 other. Of these, one is a negative image, in which the several 

 constituent parts of an object re-present themselves geometrically, 

 by virtue of the unequal emergence of light which is caused by 

 their mass affecting unequally the transmission of the incident 

 rays. This image may, for shortness sake, be called the 

 *' absorpiwti image,'" because partial absorption is the principal 

 cause of the different amount of emergent light. It is the bearer 

 of the ''defining" power, whose amount is determined by the 

 greater or less exactitude w4th which direct incident light is 

 brought into perfect homofocal reunion, the condition under 

 which images of this kind are produced. Consequently, it is 

 always the direct light— just as it comes from the illuminating 

 source — which " defines," no matter in what direction it arrives 

 at the objective, i.e., whether the central or peripheral zones of 

 the objective receive it. But, independently of the ^^ absorption 

 image," all such parts of the object as contain interior structure 

 will be imaged a second time, and this time as a positive 

 image, because these parts will appear as if self-luminous, 

 in consequence of the diffraction phenomena which they 

 cause. This second image which may be called the ''diffraction 

 image," consists, strictly speaking, of as many partial images as 

 there are separate diffraction pencils entering the objective since 

 each of these produces a positive image, as shewn in the 

 experiments before mentioned. But as these partial images, 

 taken singly, are void of content, and visible details appear first 

 only when two or more of them blend together, the total effect 

 {i.e,, their fusion into one image) is that which must be 

 practically regarded as the independent factor. Now this 

 "diffraction image" is manifestly the bearer of "resolving" 

 power, that is, the discriminating or separating faculty of the 

 microscope. Its development depends, therefore, in the first and 

 chief place upon angular aperture, in so far as this alone 

 determines, according to rules above given, the limits of its 



