Mr. J. P. Cooke on a Spect?*oscope. Ill 



The general construction of the instrument is shown in 

 fig. 1. 



The two telescopes are constructed in the usual way. The 



Fie. 1. 



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 tele- 

 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 diverging from the slit and rendered 

 parallel by the object-glass of the collimator next pass through 

 a series of prisms adjusted around a conical wheel, which will 

 be soon described, and are then received by the telescope B. 

 The spectrum, which is formed at the focus of the object-glass 

 of this telescope, is examined with eyepieces of different mag- 

 nifying powers in the usual way. The object-glasses of the tele- 

 scopes are 2^ inches in diameter, and have a focal length of 15^ 

 inches. They rest in Y's, and are provided with spirit levels and 

 adjusting- screws. The frames which hold the telescopes are 

 supported on pivots turning in sockets at the ends of two arms 

 connected with the body of the instrument, and may be clamped 

 in any position. The arm which carries the collimator is per- 

 manently attached to the main iron plate, but the arm which 

 carries the observing-telescope may be moved around the 

 plate. 



The details of the construction are shown in tig. 2, which is 

 a section made through one of the legs of the tripod and the 

 moveable arm, the telescope with its frame and pivot having first 

 been removed from the socket. (This figure, as well as fig. 4, 



