118 Mr. J. P. Cooke on a Spectroscope, 



given on page 115, is precisely the same (267° 37' 50"); and the 

 sum of those measured on the edge of the plate (267° 37' 30") 

 only differs from this by 20". 



When it is not important to use absolutely the whole aper- 

 ture of the prisms, it is not necessary to change the position of 

 the collimator in passing from one part of the spectrum to an- 

 other. If we adjust the collimator as above described, when the 

 prisms rest against the middle circle on the wheel, the whole 

 spectrum can be passed under review with great rapidity with- 

 out any further change of the collimator, and each point seen 

 under the condition of minimum deviation. When, however, 

 on account of the feebleness of the light, it is important to use 

 the whole aperture of the prisms, a slight gain can be obtained 

 by readjusting the collimator at the extreme points of the spec- 

 trum. In making the measurements described in this paper, an 

 important advantage is gained by keeping the position of the col- 

 limator fixed; for if its position is changed, the point to which 

 the angular measurements are referred is changed also, and must 

 be determined anew. If, however, the arms to which the tele- 

 scopes are attached are so arranged with a sliding motion that 

 both the collimator and the observing-telescope may be moved 

 parallel to themselves without altering their relative angular posi- 

 tion, the whole aperture of the prisms may at any time be used, 

 and nevertheless all the measurements referred to the same point 

 on the graduated limb. 



Besides the set of glass prisms, the Cambridge spectroscope is 

 also provided with a set of sulphide-of-carbon prisms. They are 

 mounted on a separate plate with a separate wheel of the proper 

 dimensions, and are arranged in all respects like the glass prisms 

 above described. The plates are provided with handles, so that 

 one battery of prisms may be quickly lifted off from the instru- 

 ment and the other put in its place. But although the liquid 

 prisms are valuable on account of their great dispersive power 

 in bringing out faint lines, especially in the more luminous por- 

 tions of the spectrum, yet their use is very restricted. One dif- 

 ficulty arises from the immensely rapid change of the index of 

 refraction of sulphide of carbon with the slightest change of 

 temperature. We have noticed within half an hour on a sum- 

 mer's day a change of 8' in the angle of minimum deviation of 

 a single sulphide-of-carbon prism of 45°, although the tempera- 

 ture of the room had only in the meantime changed eight-tenths 

 of a Centigrade degree. The temperature of the prism un- 

 doubtedly changed much more than this ; but when we remem- 

 ber that the variation thus produced would amount to over one 

 degree for the nine prisms, it will be seen that they must be use- 

 less for any purposes of direct measurement. Again, sulphide 



