116 



Mr. J. 1\ Cooke on a Spectroscope. 



moving, of course, the prism on the plate without disturbing 

 the position of the plate itself. In like manner, other prisms 

 may be added until the required number is obtained. (In the 

 Cambridge spectroscope there are nine glass prisms of 45°, as 

 shown in fig. 4.) This adjustment has only to be made, however, 

 once for all, since, 



when the prisms v^* 



are adjusted, they 

 are fastened to a 

 very thin flexible 

 brass ribbon, which, 

 passing through a 

 box at the back of 

 each prism, is there 

 secured by clamp- 

 ing - screws. The 

 ends of this ribbon, 

 moreover, are at- 

 tached to a small 

 brass drum, which, 

 being wound up by 

 an ordinary clock- 

 spring, draws the 

 chain of prisms 

 tightly around the 

 conical wheel, and 

 keeps them always 

 in place. By tra- 

 cing the path of a 

 ray of homogene- 

 ous light through 



a series of similar prisms, as is shown in fig. 4, it will be found 

 that the path of the ray within the prisms is always a tangent to 

 the same circle, provided that it passes through all under the 

 conditions of least deviation. Assuming, then, that the dis- 

 tances between the prisms are invariable, as they must be when 

 the prisms are fastened to a brass ribbon as just described, it 

 will be evident, from a moment's reflection, that, the greater the 

 refrangibility of the given ray, the less must be the diameter of 

 the circle around which the prisms should be arranged in order 

 that the ray may pass under the required conditions ; and know- 

 ing the dimensions of the prisms as well as the index of refrac- 

 tion and dispersive power of the glass, it is easy to calculate ap- 

 proximately what the diameter should be in a given case. 

 The dimensions of the conical wheel A, fig. 2, were thus deter- 

 mined — the largest diameter, 9J inches, corresponding to the 



