98 PROCEEDINGS OF THE 



illumination wMch is obtained when tlie image of the source 

 corresponds point for point with the object, and therefore 

 logically requires the use of a condenser as perfectly corrected as 

 the objective. He reached the conclusion that the ideals of 

 critical illumination appeared to have no justification on the basis 

 of the Abbe theory ; that its ideals were not realised, as difiraction 

 spectra are formed in the upper focal plane of the objective ; and 

 that, owing to the inevitable presence of difiraction effects, they 

 were unrealisable. The success of critical illumination appeared 

 in Dr. Hartridge's opinion to be due to its failure to achieve its 

 ideals, and that it had survived because of its imperfections. The 

 supposed superiority of critical illumination having been shown to 

 be illusory, Dr. Hartridge investigated some other methods which 

 appeared promising with a view to finding out if any were equal 

 to it in practice. The following three methods were com- 

 pared by experiment : 



(1) Critical illumination in which an image of the illuminant is 

 focused on to the object slide. 



(2) The method in which an image of the illuminant is focused 

 into the lower focal plane of the condenser by means of a bull's- 

 eye. 



(3) Sir A. E. Wright's method, in which a piece of opal glass is 

 placed at the lower focal plane of the condenser and illuminated 

 from behind. 



The following tests were applied : (1) The resolution of a 

 grating of 14,000 to the inch with a 4:0-mm. objective. (2) The 

 resolution of P. angulatum (40,000 lines or dots to the inch) with a 

 4-mm. objective of N.A. 0-95. (3) The resolution of A. pellucida 

 into lines and dots (100,000 lines to the inch) with an apochromat 

 of N.A. 1'4. In the first two cases a Davis shutter was used 

 behind the objective, and this and the substage diaphragm were 

 closed until resolution began to break down in some parts of the 

 field. The apparatus was arranged so that the change of illumina- 

 tion could be rapidly made without altering the adjustment. No 

 alteration whatever in resolving power could be detected when 

 this was done. Dr. Hartridge then discussed the various 

 advantages and disadvantages of the three methods. The 

 chief disadvantage of critical illumination appears to be that 

 it is almost impossible to obtain uniform illumination of the 

 object, an essential feature for photography, and that it is 



