J. P. Cooke on the Spectroscope. 805 
Art. XXXI.—On the Construction of a Spectroscope with a num- 
ber of prisms, by which the angle of minimum deviation Jor any 
ray may be accurately measured and its posi Log wn the solar spec- 
trum determined ; by JostsH P. Cooke, Jr. 
In an extract from a letter of the author published in this 
Journal, vol. xxxvi 
The two telescopes are constructed in the usual way. The 
telescope A, which we shall call the collimator, has an adjust- 
able slit placed exactly at the focus of the object glass. The 
_ Small tube which carries the slit slides into the body of the tel- 
_ €scope, but a rack and pinion motion would be preferable, so 
_ that when the focus is changed the slit will necessarily remain 
Vertical. The rays of light Sveriag from the slit and rendered : 
Parallel by the object glass of the collimator next pass fro 
