76 HAMILTON HARTEIDGE ON 



Section II, 

 The Position op Critical Illumination. 



It has been held by many experienced microscopists, and by 

 the British school in particular, that critical light difiers from 

 illumination of every other kind when applied to the microscope, 

 and that the images produced only reach perfection when critical 

 light is employed (Nelson). As is well known, critical illumina- 

 tion is obtained when the image of the source corresponds point 

 for point with the specimen under examination. The source 

 should be small in order that the observer should know when an 

 accurately focused image has been formed in the plane of the 

 specimen. The use of a condenser with definition as perfect as 

 that of objectives is logical, since the correction of spherical and 

 chromatic aberration is of the highest importance if the image 

 of the source and the specimen are to correspond point for point. 

 Assuming that a perfect condenser lens system can be obtained, 

 three questions arise for consideration : firstly, to what extent 

 are the ideals of critical illumination compatible with the Abbe 

 theory ; secondly, are its ideals realisable ; and thirdly, can they 

 be realised in practice ? 



With regard to the first question. According to the Abbe 

 theory, the source is so situated that plane waves illuminate the 

 object. If on the other hand with critical illumination point-for- 

 point illumination of the object is to be realised, then the source 

 must be so situated that convergent waves from it meet at points 

 coincident with the plane of the specimen. The source on the 

 basis of the Abbe theory cannot therefore be identified with that 

 required for critical illumination. Further, according to the 

 wave theory of light, permanent and therefore visible interference 

 can only occur between waves which have proceeded from the 

 same source, or from sources in constant phase with one another. 

 And further it is found by experiment that, in the case of most 

 ordinary sources, interference can only occur if the light waves 

 have proceeded from the same part of the source. Combining 

 the above, we find that the different parts of a source radiate 

 independently of one another. Now if in critical illumination a 

 perfect image of the source were formed on the specimen, then 

 one part of the smallest specimen receives light which bears no 

 phase relationship to that falling on any other part, and there- 



