298 E. M. NELSON ON METHODS OF ILLUMINATION. 



quality, is not much in favour with microscopists. Glass is 

 simpler, more portable and cleaner. 



Screens for sunlight with heliostat, for oxyhydrogen light and 

 for photographic purposes are different, and are beyond the limit 

 of this article. 



Spectrum. There is another method of illumination, viz. that 

 of using the spectrum, or rather a portion of the spectrum, itself ; 

 this I have described elsewhere, so will not repeat. A prism 

 spectrum is better for this purpose than a grating, for a prism 

 forms only one spectrum, and all the light which is dispersed 

 goes into it ; on the other hand, a grating makes several spectra, 

 and, as only one of them can be used, much light is lost, but the 

 dispersion of a Rowland's 14,400-line grating between E and G 

 lines in a spectrum of the first order is more than double that of 

 an ordinary flint prism. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XL, No. 69, Noveuibtr 1911. 



