ON THE THEORY OF THE MICJROSCOPE. 239 



preied according to stereometric rules, adttiits of peifecilij safe 

 inferences respecting its morpliologic constitutioyi. On the other 

 hand, all minute structures whose elements lie so close together as to 

 occasion noticeahle diffraction phenomena will not he geometrically 

 imaged, that is to say, the image will not be formed, point for 

 point, as usually described by the re-union in a focal point (or 

 plane) of pencils of light which, starting from the object, undergo 

 various changes of direction in their entrance and passage through 

 the objective; for even when the dioptric conditions requisite for 

 such a process are fulfilled, the image so formed shews none of 

 the finer structural detail, unless at least two of the diffraction- 

 pencils which are caused by the splitting up of rectilinear rays are 

 re-united. 



i^ow to any one who clearly realises in his own mind what are 

 the assumptions upon which a similitude between an object and 

 its optical image is ct)mmonly accepted, the foregoing facts must 

 suffice to lead to the concliision, that under the circumstances 

 above indicated such acceptance is a purely arbitrar}^ supposition. 

 As a positive instance of the contrary stands the conclusion to 

 which experiments 4 and 5 lead by rigorous deduction, namely, 

 that different structures always yield the same microscopic images as 

 soon as the difference of diffraction-effect connected ivith them is 

 artif daily removed from the action of the microscope ; and that 

 similar structures as constantly yield different images when the 

 diffractive effect taking place in the microscope is artificially rendered 

 dissimilar. In other words, the images of structure arising from 

 the operation of the diffractive 'process stand in no constant relation 

 with the real constitution of the oljects causing them, hut rather with 

 the diffraction phenomena themselves, which are th: true causes of 

 their formation. As this is not the place to enter into a physical 

 exposition of such phenomena, it may suffice to say in brief, that 

 the conclusions here deduced from facts won by direct observa- 

 tion, are fully substantiated by the theory of undulation of light, 

 which shews not only why microscopic structural detail is not 

 imaged according to dioptric law, but also how a different process 



