AND ON THE ABSORPTION OF THE PRISMATIC COLOURS. 439 



1. Dr Young {Phil. Trans. 1803) has long ago remarked, that 

 the light transmitted by a specimen of Blue glass exhibited two 

 red spaces when analysed by a prism ; but he does not mention 

 their relative intensities. Among the various kinds of blue glass 

 which I have examined, I ha%"e found one, which, like Dr 

 Young's, leaves two red spaces, or rather an orange-red and a 

 red space, the rays which belonged to the intermediate portion 

 having been entirely absorbed, and the least refrangible of the 

 red spaces being more luminous than the other. In another 

 specimen, the interior orange-red image was absorbed along 

 with the middle one, and in a third specimen, the exterior 

 red image and the middle one were alone absorbed. In all 

 these glasses the absorbent power attacks also the green rays 

 adjacent to the red, and leaves a greenish-yellow image, sepa- 

 I'ated by a dark interval from the most refrangible green. 

 When light is reflected from the blue oxidated surface of steel, 

 the middle red space is completely absorbed, and there remains 

 only of the inner orange-red space, a few of the red rays that 

 border upon the green light. From these experiments it fol- 

 lows, that blue glass attacks the spectrum in several points at 

 the same time, and after absorbing all the middle rays, it leaves 

 only the extreme red, and the portions of the blue and violet 

 spaces which are contiguous. By increasing the thickness of 

 the glass, however, the violet is at last overpowered, and the 

 red alone remains. The very same phenomena are obtained 

 by fluid media of a blue colour. With some Blue glasses, 

 however, the red is overpowered before the violet. See Fig. 4., 

 Nos. 2, 3, 4, 5. 



In making a series of analogous experiments with Green me- 

 dia, I have found that they attack the spectrum at both ex- 

 tremities, but the blue end with more force than the red end. 

 By increasing the thickness of the glass, the blue and red light 

 gradually diminish, but it is extremely diflScult to free the 



VOL. IX. p. II. 3 k erreen 



