igis-] 



OF A TEN-INCH DIFFRACTION GRATING. 



141 



serious difficulty is encountered in an attempt now in progress to 

 produce gratings of twenty inches or more. Such a method may be 

 made partly or perhaps completely automatic, and would be inde- 

 pendent of screws or other instrumental appliances. 



Enlargement of photograph of the green mercury line \ 5461, taken 

 by H. L. Lemon with lo-inch dififraction grating in sixth order. Scale : i 

 division = o.oi A.U. ; ruled surface 9^ in. X 2^ in., 11,700 lines per inch. 

 Mounted in Littrow form with 8-inch lens by Brashear. Focal length 20 feet. 



It may be pointed out that an even simpler and more direct 

 application of Hght-waves from a homogeneous source is theoretic- 

 ally possible and perhaps experimentally realizable. 



If a point source of such radiations send its light-waves to a 

 collimating lens and the resulting plane waves are reflected at normal 

 incidence from a plane surface, stationary waves will be set up as 

 in the Lippman plates ; these will impress an inclined photographic 

 plate with parallel lines as in the experiment of Wiener ; and the 

 only limit to the resolving power of the resulting grating is that 

 which depends on the degree of homogeneity of the light used. As 

 some of the constituents of the radiations of mercury have been 

 shown to be capable of interfering with difference of path of over 



