308 J. P. Cooke on the Spectroscope. 
may be placed on the outer rim of the plate, and if only the ra- 
dial arm, which carries the observing telescope, moves concen- 
tric with the graduated are, the axis of the telescopes themselves 
may make any angle with the radius whatever. Let Op and 
Og be two radii of the graduated circle. Let Ap and Bg rep- 
resent the axes of the two telescopes in collimation and making 
any undetermined angles with the two radii. Place now a prism 
at m and turn the radial arm Og into the position Og’, but with- 
out changing the inclination of the axis of the telescope to the 
arm, and let BmB’ be the angle of minimum deviation. Since 
now the two mriangles gsm and g‘so are similar, it is evident 
that the angle Bm B’ is equal to the angle goq’, and is therefore 
ime by the are intercepted between the radii Og and Og’. 
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iron place C fig. 2. Two brass pins were also attached to the 
Sectrix of the refracting angle should coincide with a radius of 
the wheel. The last adjustment was made with the aid of a 
