MICROSCOPIC ILLUMINATION. 79 



for observing tlie adjustment of the flame image in tlie lower 

 focal plane of tlie substage condenser. 



(The use of an aplanatic lens system, and the careful adjust- 

 ment for obtaining plane-wave illumination may seem to be 

 at variance with what I have stated above, namely, that for 

 practical purposes the employment of plane-wave illumination 

 appears to be unnecessary. These precautions were used because 

 I was not convinced, at the time these experiments were made, 

 that plane-wave illumination could be dispensed with. This 

 system of illumination is that which Ainslie (2) has called system 

 A, because he believed it to have been originated by Abbe for the 

 purpose of obtaining illumination with plane waves. Ainslie 

 has pointed out that while the method itself is valuable, the 

 importance of plane- wave illumination is doubtful, and probably 

 the best procedure in practice is to focus the image of the source 

 on to the plane of the iris diaphragm of the condenser, wherever 

 that may be.) 



The substage condenser used was a Conrady " Holos " oil- 

 immersion system of 1'35 N A., this combination being chosen 

 because of its high correction. Tests showed, however, that 

 there were residual amounts of under- correction, both for aplana- 

 tism and chromatic aberration. The usual test for aplanatism 

 (namely with the flame image in focus on the specimen to examine 

 whether the back lens of the objective appears completely filled 

 with light) is unreliable because it varies with the size of the 

 flame. An alternative and better test is to watch for movement 

 in the image of a small source focused on the slide when a slit- 

 shaped aperture is moved below the condenser, so as to expose 

 different condenser zones. This method is analogous to that 

 which I have described in a separate paper under " tube-length 

 adjustment" (7). Details of the technique will be found de- 

 scribed there. 



This test, which is very searching, showed that the condenser 

 was under- corrected for spherical aberration in the peripheral 

 zones. This could be eliminated by methods similar to those 

 applied in the case of objectives, that is by increasing the optical 

 tube length, or by separating the components of the lens system 

 (equivalent to using a correction collar). An auxiliary positive 

 lens system was therefore placed below the condenser which was 

 chromatically over-corrected, eliminated almost completely the 

 JouRN. Q. M. C, Series II.— No. 85. 6 



