J. P. Cooke on the Spectroscope. 307 
concentric with the graduated limb and bears a vernier by which 
he angular motion may be determined, reading to 10”. In 
the socket of the first plate rests the pivot of a second plate, 
C, which turns on the first and supports the prisms with the ad- 
justing wheel A. The diameter of the upper plate is an inch 
less than that of the lower plate, so as to expose the graduated 
arc near the outer edge of the latter, and its upper surface is as 
flat and even as possible. Rising from the center of the upper 
plate there is a tall screw pivot of iron, B, on which turns a 
conical wheel, made also of iron. By this motion the wheel 
may be either raised or lowered. This wheel.is am essential por- 
tion of the instrument, and we will next consider the theory 
of its use. 
of the telescope. The position of the vernier on the graduated 
arc is now Lae Then, having adjusted the prism, both the 
_ It will be obvious however 
from fig. 3, that it is not neces- 
Sary for the accuracy of t 
Measure either that the prism 
Should be placed at the center 
