in Spectrum-an 



and Distortion of the Sped? 



367 



<p. 



H. 



Diff. of fi. 



5°7 ' 

 57 7 51 



57 30 

 58 



58 30 

 59 



59 30 

 60 



60 30 

 61 



61 30 

 62 



62 30 

 63 



63 30 

 64 



64 30 

 65 



65 30 

 66 



66 30 

 67 



67 30 

 68 



68 30 

 69 



69 30 

 70 



1-680148 

 1-680133 

 1-680125 



132 



1-679993 



243 



1679750 



352 



1-679398 



464 



1-678934 



573 



1-678361 



683 



1-677678 



794 



1-676884 



902 



1-675982 



1013 



1-674969 



1122 



1-673847 



1-672623 



1224 



1374 



1-671259 



1435 



1-669824 



1547 



1-668277 



1681 



1-666596 



1800 



1-664796 



1861 



1-662935 



1-660941 

 1-658840 



1-654313 



1-649359 



1-643999 





From this Table it is easy to construct a curve which shall 

 represent to the eye the relations of //, and cf> ; and either from 

 the curve, or directly from the Table, to show to what extent 

 the spectrum is distorted. The scale given on the following 

 page is intended to illustrate this. The left column of the scale 

 represents an arc graduated to half degrees, along which moves 

 the index attached to the handle by which the prism is turned 

 round. The degrees marked on the scale indicate the angle of 

 incidence of the light on the first face of the prism. It might 

 be convenient, however, to graduate this arc from the central 

 line of minimum deviation (very near the line E) as zero. The 

 right column of the scale gives the corresponding values of the 

 refractive indices. It would be convenient to mark on one of 

 the two scales the positions of the principal lines in the spectrum. 

 This might be done either by experiment, or from the value of 

 the refractive indices into bisulphide of carbon of the fixed lines, 

 if they have been determined. 



This distortion of the spectrum, or apparent exaggeration of 

 its central portions, may be made evident to the eye by turning 

 the prism slowly round with a uniform motion by moving the 



