88 HAMILTON HARTRIDGE ON MICROSCOPIC ILLUMINATION. 



consequent damage to the bull's-eye. It is, however, generally 

 possible — with high powers at any rate — to obtain a large 

 illuminated field without special difl&culty in this respect. 



Ainslie further points out that if the distance between con- 

 denser and bull's-eye is increased with a view to obtaining a large 

 enough image of the light-source to fill the aperture of the sub- 

 stage iris, it is well to have the " bull's-eye iris " separately 

 mounted, so that its distance from the condenser may be varied 

 to suit the thickness of the object slip. 



May I take this opportunity of thanking Commander Ainslie 

 for the valuable suggestions that he has made to me during the 

 revison of this paper. 



Summary. 



(1) The source from which the rays are traced, according to the 

 Abbe theory, is only in exceptional cases the source of light. 



(2) The ideals of critical illumination are contrary to the 

 Abbe theory, and if obtained would destroy resolution. 



(3) The ideals of critical illumination are unobtainable in 

 practice. It is because of this that the technique of critical 

 illumination has survived. 



(4) With a carefully designed optical system for obtaining 

 critical illumination, the minimal working aperture required to 

 resolve certain test objects was ascertained. No decrement in 

 the resolving power could be found when two other systems of 

 illumination were substituted. 



(5) One of these systems provides a very simple technique for 

 applied microscopy. The other has marked advantages for 

 photo micro gr aphy. 



(6) A fourth method is found to give very good results and to 

 be convenient in practice. Details of this method are described 

 and illustrated. 



References. 



1. Spitta. Microscopy, 1907, p. 401. 



2. Ainslie. Proc. Phototnicro. Soc, 1918, p. 1. 



3. Johnstone Stone y. J own. Roy. Micro. Soc, 1897, p. 71. 



4. Conrad Y. Journ Roy Micro. Soc, 1904, p. 631. 



5. Ainslie. Journ. Qiiekett Micro. Soc, 1915, p. 561. 



6. Conrad Y. Journ. Roy. Micro. Soc, 1904, p. 612. 



7. Hartridge. Journ. Roy. Micro. Soc, 1919, p. 119. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XIV., No. 85, November 1919. 



