274 Mr. Hunt ow Light which has permeated coloured Media, 



19. It will be observed, that the light which has passed 

 through a green medium (2, 7, 9, 10, 15, 16) acts less 

 powerfully in darkening photographic papers, and occasions 

 vegetable leaves to be even paler than that which has been 

 subjected to the interference of a yellow medium. 



I am led to suspect that the band of rays formed by the 



meeting of the yellow and the green has an influence similar to 



the extreme red, in neutralizing the powers of the other ad- 



j acent rays, as was first noticed by Sir John Herschel, (22.), 



(23.), (26.). 



20. The figure on the preceding page represents the solar 

 spectrum, as it impresses itself on a daguerreotype plate, not in 

 shadows merely, but in colours, which have the peculiar ap- 

 pearance of the down upon the nectarine. 



The most refrangible portion of the spectrum is repre- 

 sented in full colours, shading from indigo to a delicate rose, 

 which is lost in a band of pure white. 



21. Beyond this a protecting influence is powerfully ex- 

 erted, and notwithstanding the chemical effect .produced over 

 the plate, by the dispersed light, a line is formed free of mer- 

 curial vapour, and which consequently appears black. 



22. The green portion of the spectrum is represented in 

 its true colour, but it is considerably less in size than the 

 space occupied by these rays. 



23. The yellow rays are without action, or rather they do 

 not prepare the silver for the reception of the mercury, and 

 consequendy a black belt marks the space on which they fell, 

 and extends a little beyond it into the green (19.). 



24. A white line marks the place of the orange light. 



25. TJie red is represented by a well-defined rose colour, 

 bounded, as were the more refrangible rays, by a white line, 

 shaded at the lower extremity with a green. 



This passing of the red into a green and of the blue into a 

 rose colour (20.) is strikingly similar to the effect produced, 

 by the interference of coloured media, on some photographic 

 drawings (8.). 



26. The lowest dark space on the picture is a beautiful il- 

 lustration of the influence of the extreme red rays in protect- 

 ing the silver from luminous action (19.) (21.). 



27. What appears more surprising to me than even the de- 

 tection of the negative ? rays at each end of the prismatic 

 spectrum, is the continuation of the dark line throughout its 

 isohole length, evidently showing the influence of the same cause 

 as is so effective at the least refrangible extremity. 



This band is not equally defined throughout its entire cir- 

 cumference. It is the most strikingly evident from the ex- 



