82 HAMILTON HARTRIDGE ON 



was found that even then the resolution into dots was to be 

 observed in parts of the field only. 



On performing these tests, I was unable to find any change 

 whatever in the resolving power of these objectives, nor were 

 details apparently changed in any of the specimens. When 

 colour filters were used in order to restrict the illumination to 

 chosen parts of the spectrum, no variation in this conclusion. 

 was found necessary. I was therefore unable to find any difierence 

 between critical illumination and the two alternative methods 

 so far as the resolution of the microscopic image was concerned. 



Apart altogether from resolving power, there is the further 

 question as to the advantages of restricting the field of illumina- 

 tion, for both Ainslie and I have found that when this is done the 

 image appears more distinct, and may be examined for longer 

 periods without fatigue. This is probably due to two causes : 



(1) A physical one — namely, less scattered light from parts of 

 the optical system between the slide and the eye. 



(2) A physiological one— namely, a reduction in the fatigue 

 of the retina, and possibly also increased contrast. 



There appear to be two ways of obtaining a restricted area of 

 illumination : firstly, by closing an iris diaphragm fitted at the 

 lower focal plane of the eyepiece (this method is independent of 

 the system of illumination) ; secondly, to restrict the illumina- 

 tion of the slide to that portion which (when magnified) will fill 

 the chosen part of the field of the eyepiece (this method necessi- 

 tates that an iris or other diaphragm be co-ordinate with the 

 slide, and therefore excludes the use of methods of illumination, 

 such as nos. 1 and 2, which do not possess this feature). Of the 

 two alternative schemes the second is the better, since the 

 elimination of scattered light is more complete. As pointed out 

 above, however, a very well corrected condenser is required in 

 order that the first method shall be definitely outclassed. 



Section V. 

 The Relative Advantages of Different Methods. 



One advantage of the opal-glass method (3) is that an un- 

 corrected condenser (Abbe type) can be employed. This may be 

 used dry or with water or oil immersion. The method is un- 

 afiected by slide thickness, since spherical aberration may be 



