Mr. J. P. Cooke on a Spectroscope. 113 



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 an essential por- 

 tion of the instrument, and we will next consider the theory of 

 its use. 



In the ordinary method of measuring the angle of minimum 

 deviation with a Babinet's goniometer, the prism is placed on a 

 revolving plate at the centre of the graduated circle, and so ad- 

 justed that the edge of the refracting angle shall be perpendicu- 

 lar to the plane of the circle, and its bisectrix parallel with a 

 diameter of the circle. The axis of the collimator and obser- 

 ving-telescope, moreover, being parallel with a diameter of the 

 circle, it is evident that, when the prism and telescope are turned 

 into the position of minimum deviation, the vertex of the angle 

 of minimum deviation will coincide with the centre of the 

 circle, and hence the arc intercepted between two radii of the 

 circle parallel to the axes of the two telescopes will be 4 the 

 measure of the angle required. This angle is practically mea- 

 sured by first bringing into line of collimation the observing- 

 telescope and collimator, so that the image of the slit at the end 

 of the collimator coincides with the vertical wire in the eyepiece 

 of the telescope. The position of the vernier on the graduated 

 arc is now noted. Then, having adjusted the prism, both the 

 prism and the telescope are turned to the position of minimum 

 deviation, and a coincidence established between the vertical 

 wire and one of the lines of the spectrum. The vernier is now 

 again read, and the difference between the two readings is the 

 angle of minimum deviation for the ray corresponding to that 

 line. 



It will be obvious, however, Fig. 3. 



from fig. 3, that it is not ne- 

 cessary for the accuracy of B 

 this measure, either that the 

 prism should be placed at the 

 centre of the circle, or that 

 the axis of the telescope 

 should be parallel to one of 

 its radii. If only the bisec- 

 trix of the refracting angle 

 passes through the centre of 

 the circle, the prism may be placed on the outer rim of the 

 plate ; and if only the radial arm, which carries the observing- 

 telescope, moves concentric with the graduated arc, the axis of 

 the telescopes themselves may make any angle with the radius 

 whatever. Let Op and q be two radii of the graduated circle. 

 Let Kp and B q represent the axes of the two telescopes in col- 



