450 ON THE ABSORPTION OF LIGHT BY COLOURED MEDIA, 



The facility with which the extreme red ray is transmitted 

 by this glass, in comparison with the rays of intermediate re- 

 frangibility, produces a change in the general hue of the glass 

 corresponding to its thickness. In small thicknesses it ap- 

 pears purely blue ; but as its thickness is increased, a purple 

 tinge comes on, which becomes more and more rudd}', and, 

 finally, passes to deep-red, a great thickness being, however, 

 required to produce this effect. 



6. On the other hand, a glass of a rich ruddy purple was 

 found to have its type, as in Fig. 5. The curve 1 correspond- 

 ing to a thickness 0.065 in. ; 2 to a thickness 0.086 ; and 3, in 

 which the least refrangible rays are almost entirely obliterated, 

 to their sum. In consequence of this, the tint of the glass 

 loses its ruddy hue, by an increase of thickness, and passes to 

 a fine violet. 



7. The extreme red light, transmitted with such facility by 

 the glass described in Art. 6., is copiously absorbed by this. 

 The species of light alluded to is remarkable, ^rst, for its per- 

 fect homogeneity ; and, secondly, for its position in the spec- 

 trum. When the solar spectrum received on a white paper in 

 a darkened room, is viewed through a moderate thickness 

 (0.08) of that glass (of Art 6.) cemented to any red glass of a 

 tolerably pure colour, it will be seen reduced to a perfectly cir- 

 cular and well-defined image, of a deep-red colour. If a pin 

 be now stuck in the centre of the red circle, it will be found, 

 on removing the glass from the eye, to have been fixed in 

 what an ordinary observer would call the very farthest termina- 

 tion of the red rays, and a mark similarly made at its circum- 

 ference will appear to lie wholly without the spectrum among 

 the dispersed light which usually hangs about its edges. In 



other 



