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ON METHODS OF ILLUMiNATlON. 



By Edward M. Nelson, F.R.M.S. 



{Bead May 2ird, 1911.) 



Mirror Illumination. By this term is meant the illumination 

 of an object by transmitted light with a plane or concave mirror 

 without any substage condenser. This form of illumination was, 

 up to a few years ago, yery extensively employed, the use of a 

 condenser, especially upon the Continent, being quite exceptional ; 

 in this country, however, condensers were more often to be 

 seen. The medical student, the histologist and biologist never 

 used any other kind of illumination ; in fact, most medical 

 students and biologists, even now, think that it was Prof. 

 Abbe who invented the substage condenser. They are not 

 aware that the achromatic condenser, in the form of an object- 

 glass, was in use before Prof. Abbe was born ; and ten years 

 after its introduction a three-lens chromatic condenser, very 

 similar to Prof. Abbe's, appeared, which failed to become estab- 

 lished here because its achromatic rival was preferred. It should 

 be noted also that a single-lens chromatic condenser had been 

 fitted to all English microscopes of the better sort since Benjamin 

 Martin's time (say 1775). There were plenty of condensers, but 

 biologists would not use them. As the history of the condenser 

 is not our subject, we will pass on at once to the mirror and the 

 manner of using it. 



Nearly all microscopes are now fitted with plane and concave 

 mirrors, but a very few elementary instruments have only a 

 concave; the plane is for use in conjunction with a substage con- 

 denser, and the concave for use by itself. 



The ideal illumination for transmitted lisrht is obtained when 

 the object is at the apices of two conjugate solid cones of light. 



