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Transactions of the Society. 
in the card being treated exactly as if it were the lamp-flame. By a 
slight rotation of the Wray lens any colour of the spectrum is made 
to fall on the aperture in the card, and by this means the required 
colour for the illumination of the Microscope is obtained. For 
resolving purposes the blue-green will probably be found the most 
suitable. 
The next question is — What do we gain by this apparatus ? In 
the first place, with regard to resolving power with blue-green light, it 
practically adds *1 to the N.A. of the objective: thus a D.D. of '8 
becomes a lens of *9 N.A., and that too without incurring any 
increase of spherical aberration. Secondly, as there can be no secondary 
spectrum, an ordinary achromatic lens performs as well as an 
apocliromatic. For photomicrographic W’ork it will be useful in taking 
the place of the coloured screens which are so necessary when 
isochromatic plates are used. For purposes of resolution, however, I 
do not think that it will prove of any assistance to photomicrography, 
Mr. Comber having pointed out that the plate itself is a monochromatic 
light-selecter. 
