270 Dr. Herschei/s Investigation of the Powers 



whether they proceeded from this mixture ; and therefore co- 

 vered the prism in front with a piece of pasteboard, having a 

 slit in it of about T r - of an inch broad. 



lotb Experiment. The nail, at 9 feet 2 inches from the prism. 

 R. I fix my attention on two shining, red points ; they are 

 pretty bright. 



O. I see many more points. The object is better illuminated 

 than in the red. The points are surrounded by black ; but are 

 orange-coloured. 



Y. The points now are yellow, and white surrounded by 

 black. The object is better illuminated than in orange. 



The maximum of illumination is in the brightest yellow, or 

 palest green. 



G. The points are green and white, as before surrounded by 

 black. Better illuminated than in orange. 

 B. The illumination is nearly equal to red. 

 I. Very indifferently illuminated. 

 V. Very badly illuminated. 



The phaenomena of the differently-coloured points being now 

 completely resolved, since they were plainly owing to the former 

 admixture of colours, and the illuminating power remaining 

 ascertained as before, I attempted also to repeat the experiments 

 upon the thermometer, with the prism covered in the same 

 manner ; but I found the effect of the coloured rays too much 

 enfeebled to give a decisive result. 



I might now proceed to my next subject ; but it may be par- 

 donable if I digress for a moment, and remark, that the foregoing 

 researches ought to lead us on to others. May not the chemical 

 properties of the prismatic colours be as different as those which 

 relate to light and heat ? Adequate methods for an investigation 



