334) Dr. Seebeck on the unequal Evolution 



approached in most of them to an angle of 60°. With some 

 of those that deviated more, experiments were made with the 

 largest and smallest angle, without any material difference of 

 effect resulting from them. 



As I shall make use in the sequel of the expressions " beyond 

 the limit of the red, or the violet," beyond the limit of colour 

 generally, I will explain here once for all, that in every ex- 

 periment mentioned I have retained the limits of colours 

 adopted by Newton and most other natural philosophers ; and 

 thus, like Herschel and other observers, placed the limit of 

 the red where the bright prismatic red ceases, losing itself in 

 a smaller edge of a fainter red ; and that I have assumed the 

 whole of the coloured spectrum as being included within the 

 boundary wherein the colours present themselves to the naked 

 eye. 



I cannot indeed confine the prismatic spectrum within these 

 bounds, as will be more fully shown below ; yet 1 did not wish 

 to deviate here, as I did not in my former treatise, from the 

 general manner of expression, because such a practice would 

 iiave rendered a comparison of my observations with those of 

 others more difficult. 



Although I think I may trust my eyes in recognizing and 

 distinguishing colours, I have nevertheless, for the sake of 

 greater security, engaged other persons with eyes sensible to 

 colours, to determine the limits of the spectrum, and found 

 their detei'minations to agree with mine. 



I have yet to observe, that the thermometer was emptied 

 every time after being used, and filled afresh for every new 

 series of experiments ; whence the inequality in the figures in 

 the experiments made at different times, which however agree 

 in all other points. 



These experiments are as follows : 



Exp. 1. — A prism of white Bohemian glass (No. l). Distance 

 of the thermometer from the prism six Paris feet. The room 

 quite dark) and a screen intercepts the prismatic colours, which 

 are not to fall on the bulb. The spectrum weiy vivid and well 

 limited. The thermoscope stands 



II III 



One inch above the violet at 12^ 



Half an inch ditto . . \ 2\ 



Rises, on the screen being lowered, to . . . . 1 2j 



In the violet 13 



blue 14 



• a little more light falling on the tube \, „j^ 



of the thermoscope J ^ 



Half in the green 2 2 



Deeper 



