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



limation, and making any undetermined angles with the two 

 radii. Place now a prism at m, and turn the radial arm q into 

 the position q', but without 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 triangles qsm and q'so 

 are similar, it is evident that the angle B m B' is equal to the 

 angle q o q 1 , and is therefore measured by the arc intercepted 

 between the radii q and q'. 



In order now to apply this principle in the spectroscope, the 

 glass prisms were mounted permanently in brass frames. The 

 frames rest on three brass pins, which were adjusted by filing 

 until the refracting edge of the prism was perpendicular to the 

 iron plate C (fig. 2). Two brass pins were also attached to the 

 back of each prism, and the lengths of these so adjusted that, 

 when the prisms were pushed against the conical wheel, the bisec- 

 trix of the refracting angle should coincide with a radius of the 

 wheel. The last adjustment was made with the aid of a gauge 

 cut from a sheet of tinned iron, fitting at the same time the 

 periphery of the wheel and the face of the prism, which was 

 applied alternately on either side. The angle of minimum de- 

 viation of the ray D was then measured for each prism in the 

 following way. 



The prism having been placed on the plate with the pins ap- 

 plied to the periphery of the wheel, the collimator was turned on 

 its pivot, and at the same time the plate C turned on its centre, 

 until, on applying the eye at the open slit and looking through 

 the object-glass towards the prism, the further back edge of the 

 prism, seen through the glass of the prism, appeared to coincide 

 with the nearer back edge seen directly. When this is the case, 

 it is evident that the rays of light which reach the eye from the 

 further back edge of the prism must pass through the glass pa- 

 rallel to the back edge of the prism, or, what amounts to the same 

 thing, perpendicular to the bisectrix of the refracting angle ; and 

 when the light passes in this way, the prism is at the angle of 

 minimum deviation. When the prism was thus placed, the col- 

 limator was turned slightly on its pivot until the axis of the tele- 

 scope prolonged passed through the centre of the prism-face, and 

 was then securely clamped in this position. This adjustment may 

 be made with sufficient accuracy by the unassisted eye. The prism 

 having now been turned on one side, the arm of the observing- 

 telescope was turned on its centre, and at the same time the tele- 

 scope turned on its pivot until it came into exact collimation with 

 the collimator. In order to facilitate this adjustment, the tele- 

 scopes are provided with caps which cover the object-glasses of 

 the telescopes with the exception of a narrow vertical slit at the 

 centre. The pivot of the observing-telescope was next clamped, 



