42 DISPERSION. 



is that occupied by the red light, ~BGcb that by the orange, 

 CDdc the yellow, DE^ 



j.i T-< ,1 * yy " g tfcSa 



the green, EF/e the /"T j j ~~j r~l r\ 



blue, FG-$/ the indigo, 1^1 i [ i j / 



and GH% the violet. *""& ^T 1 - a - 6~s i"'' 

 He then found that, if the whole length of the rectilinear 

 side, AH, be taken as unit, the distances to the confines of 

 the several colours, AB, AC, AD, &c.,will be denoted by the 

 numbers , , , i, f , f Now the intervals AB, BC, CD, 

 &c., occupied by the several colours in the spectrum, will be 

 to one another as the corresponding variations of the index 

 of refraction. If, therefore, the whole variation of /u, or -^9 

 be divided as the line AH is in the points B, 0, D, &c., the 

 refractive indices of the rays at the confines of the several 

 colours will be as follow : 



77 77| 77^ 77^- 77 77| 77| 78 

 50' 50' 60' 50' 50' 50 ' To ' 50' 



771 



The mean refractive index is ~, or 1 '55; and it appears 



from the preceding that it belongs to the rays at the confines 

 of the green and blue. 



A much more exact method of determining the refractive 

 indices of the species of simple light will be explained pre- 

 sently. 



(51) The intensity of the light is very different in the 

 different parts of the spectrum. According to the experi- 

 ments of Fraunhofer, the following numbers represent the 

 intensities of the light in each of the coloured spaces, the 

 maximum intensity (which occurs at the confines of the 

 yellow and orange) being represented by 1000 ; viz., red, 

 94; orange, 640; green, 480; blue, 168; indigo, 31; 

 violet, 6. 



