E. M. NELSON ON METHODS OF ILLUMINATION. 295 



this should not be forgotten at the expense of good definition. 

 There is one more point before the trial of this kind of illumina- 

 tion is exhausted. It may be urged that while ground glass 

 does not give such a good image as that obtained with a first-rate 

 condenser, yet, if the condenser is a bad one, it will improve the 

 image by neutralising some of its defects, such as chromatism and 

 errors of centricity. To determine if this were so a very cheap 

 condenser, consisting of two single plano-convex lenses, was tried 

 with an achromatic | in., with and without ground glass, and 

 the image was found to be better without the ground glass. In 

 the face of all these experiments Mr. Lewis Wright's predilection 

 for this method of illumination astonishes one, for he did person- 

 i\\\y work with the microscope, and knew about critical images ; 

 on the other hand Dr. Carpenter's microscopy was of the 

 biological kind, which had nothing more in it than putting it 

 under, much in the same way as one would put a photograph in 

 ^ stereoscope, so his advocacy of this imperfect method of work 

 will cause no surprise. 



Screens. This subject would not be complete without a note 

 -on screens.* Formerly we all made a mistake by pitching our 

 illumination too high up in the spectrum (I am now speaking of 

 visual, not of photographic work). There can be no doubt about 

 this, because fine detail is lost if the light is too high i up in the 

 spectrum. The cause is probably a physiological one. Experi- 

 ments show that a normal eye is more sensitive in picking up 

 fine detail when the light is a peacock green. Although with 

 light high up in the spectrum the resolving power of a lens is 

 increased, yet the sensibility of the eye is diminished. The art 

 is to strike a happy mean between the two. Few microscopists, 

 if any, have experimented more during the last thirty years with 

 screens than I, and the following are three of the lessons I 

 have learnt : 



1. Not to work with light too high up in the spectrum. 



* Screens were first introduced by Sir D. Brewster in 1836. His serene 

 was a red one. 



