!»4 0. Barus and M. Barus — Plane Grating Similar to 



The greater part of the visible spectrum is thus contained 

 between i = 15° and i = 20°. It follows that the excess of 

 p cos i — p lies between 7 and 1 .">""". Hence the eye-piece 

 may be placed at a mean position corresponding to 10""" and 

 give very good definition of the whole spectrum without 

 refoeusing, as I found by actual trial. Within i cm the focus 

 is sharp enough for most practical purposes. If the distances 

 p Q and p ' are selected so that eye-piece and slit just clear each 

 other the definition is qnite sharp. 



The diffraction equation is not modified and if 2x corre- 

 sponds to the positions -f- i and — i for the same spectrum line, 



2A = (D/B) 2x. 



It is, therefore, not necessary to touch the eye-piece, and this 

 is contributory to accuracy. 



Fig. 6. 



If Rowland's equation is differentiated relatively to p and p' 



— dp = ( — jB. ) do ' , where the dpJ '/p 2 factor is constant. 



\p cos ^/ 



Hence — dp varies as (p/cos if, given in the table. If further- 

 more a comparison is made between dp and dp this equation 

 reduces to 



\/dp /dp = [(R—p )(l—cos i)]/B cos i 



which becomes unity either for i = or for p = R (Rowland's 

 ease). 



12. Summary. — By using two slides symmetrically normal 

 to each other and observing on both sides of the point of 



