356 0. A. Young—Spectroscopic Notes. 
g grating. 
If the angle, between the normal to the grating and thre 
view-telesco e, 1s less than that between the actrisl and the 
n. 
e mathematical theory is very simple. Suppose the colli- 
mator and telescope to be fixed at a constant angle, as in the 
now usual arrangement. 
