172 Mr. Herschel on the Action of [March, 



angular separation of the axis of each colour from that of the 

 extreme red by the direct homogeneous light of a sunbeam, 

 separated by the prism, and received on the reflector of a divided 

 apparatus, when, after the proper reductions for refraction and 

 dispersion, the results were as follow : 



Colour. 



Extreme red . . 

 Mean red. .. .. 

 Mean orange. . 

 Mean yellow. . 

 Mean green..., 

 Mean blue . . . . 

 Mean indigo . . 

 Indigo violet. . 

 Mean violet. .. 

 Extreme violet 



^iPr"r?.:-Realsep„.ti, 



ration of the 



00 0' 



1 S3 



37 

 



49 

 2 



10 21 



11 17 



13 5-S 



15 23 



1 2' 



1 44 



2 40 



52 

 21 



6 54 



7 31 

 9 17 



10 14 



No. of 



obser. 



vationg. 



13 

 45 

 18 

 20 

 16 

 13 

 33 



2 



2 



8 



Though the total separation of the red and violet axes in this 

 table so far exceeds what we had before estimated it at, I am 

 fully satisfied that it is no way exaggerated, but rather falls short 

 of the truth. It is very practicable, by combinations of coloured 

 glasses, liquids, &c. to insulate either extremity of the spectrum 

 in a state of the most absolute purity. In this climate, the dis- 

 persed Hght of the sky in the neighbourhood of the sun, which 

 always mixes with the prismatic beam, is so considerable as to 

 obliterate the feeble rays which compose the two extremities of 

 the spectrum, and it is only by interposing such combinations 

 between the eye and the Iceland crystal used to analyze the 

 polarised ray, that they can be examined with any certainty. 

 The combination I employed for the extreme red was such that 

 when the whole spectrum thrown on a white screen was viewed 

 through it, it was seen reduced to a perfectly circular, well 

 defined, deep red image, whose centre fell oh the very furthest 

 termination of the red as seen by the naked eye, and whose cir- 

 cumference attained, or perhaps surpassed the point where the 

 maximum of the calorific rays has been supposed to be situated. 

 In like manner, when the same spectrum was examined with the 

 violet combination, a very slightly elongated violet image became 

 perceptible, but every trace of the indigo, and the brighter por- 

 tion of the violet rays, was extinguished. For observations on 

 the indigo, and all the more refrangible portion, I employed 

 similar artifices, without which I found it perfectly impracticable 

 to obtain any regular and comparable results. 

 I — V . 



The coefficient — - — in our formula being the only part not 



immediately deduced from observation, it is evident that the 



I c 



assumption 7 = - must be widely erroneous in the present 



