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LXVIII. On the Effect of the Position of the Grating [or Prism) 
upon the Resolving Power of a Spectroscope. By Alfred 
W. Porter, B.Sc, Fellow of and Assistant-Professor of 
Physics in University of London, University College*. 
THIS paper is an extension of an interesting paper by 
Mr. H. Morris-Airey in this Journal for March 1906 
(p. 414). In that paper it was shown that even wide slits 
may be employed if the light fall at nearly grazing incidence 
upon a grating or prism. But the closeness together of two 
spectral lines of different wave-length also varies with the 
position of the grating ; and it is necessary to examine the 
variation of this proximity before it can be definitely decided 
that a gain in purity is obtained by inclining the grating or 
prism. 
We will consider in detail the case of a grating : — 
Let 4> he the angle of incidence of the light from the colli- 
mator upon the grating and 6 the angle of diffraction ; both 
measured positive when on the same side of the normal. 
Fig. 1. 
Then, if e is the grating-interval, and n the order of 
spectrum referred to, 
e (sin 6 -f- sin <£) = n\. 
The effect of the flniteness of the width of the slit is allowed 
for by considering a small variation in the angle of incidence 
* Communicated bv the Author. 
