of the prismatic Colours to heat and illuminate Objects. 



3 / U 



that pale green light excels in illumination, the method which 

 ought to be pursued in the construction of a darkening appa- 

 ratus was sufficiently pointed out ; and nothing remained but 

 to find such materials as would give us the colour of the sun, 

 viewed in a telescope, of a pale green light, sufficiently tem- 

 pered for the eye to bear its lustre. 



To determine what glasses would most effectually stop the 

 red rays, I procured some of all colours, and tried them in the 

 following manner. 



I placed a prism in the upper part of a window, and received 

 its coloured spectrum upon a sheet of white paper. Then I 

 intercepted the colours, just before they came to the paper, 

 successively, by the glasses, and found the result as follows. 



A deep red glass intercepted all the rays. 



A paler red did the same. 



From this, we ought not to conclude that red glasses will 

 stop the red rays ; but rather, that none of the sun's light, after 

 its dispersion by the prism, remains intense enough to pass 

 through red glasses, in sufficient quantity to be perceptible, 

 when it comes to the paper. By looking through them directly 

 at the sun, or even at day objects, it is sufficiently evident that 

 they transmit chiefly red rays. 



An orange glass transmitted nearly all the red, the orange, 

 and the yellow. It intercepted some of the green ; much of the 

 blue ; and very little of the indigo and violet. 



A yellow glass intercepted hardly any light, of any one of 

 the colours. 



A dark green glass intercepted nearly all the red, and partly 

 also the orange and yellow. It transmitted the green ; inter- 

 cepted much of the blue ; but none of the indigo and violet. 



