I 



ON THE CHEMICAL. ACTION OF THE SOLAR RADIATIONS. 69 



from the reduction of the green space) ; the illuminating powers of the out- 

 standing rays are but very slightly diminished. 



(72) 60. Chemical action commences in the mean yel- Fig- 35. 

 low ray about '20 above a! ; it extends, in the first instance, 

 over a space equal to '10, forming a patch of a semi-metal- 

 lic lustre with an olive-grey colour ; this action is continued ~ | 

 for another equal space, but the impressed space has more 

 of a brown hue ; these gradually blend into one nearly cir- 

 cular spot. From about '30 above a', a second action com- 

 mences, independently of that already described ; and indi- 

 cating, as it appears to me, a set of rays of distinct character. 

 Beyond this, at about '60, another oval forms, which con- 

 tinues and extends to "15, or sometimes, if the sun is very ^ 



bright, to '20 beyond a. This space, equal to -60, is in 

 every respect very broadly distinguished from that which is produced between 

 the lines C and F of Fraunhofer. It is characterized by a light, cloudy 

 brown colour, which deepens a little in colour beyond the luminous Ym. .36. 

 rays of the ordinary spectrum, when it is somewhat suddenly shaded 

 off. The action may be well represented by two ovals, one consider- 

 ably larger and longer than the other, which overlap ; and it would 

 appear that the change of colour observed in the upper section of the 

 lower space is due to this involved action of two sets of rays. In the 

 chemical spectrum described (52), obtained after the absorptive action 

 of a medium yellow glass (6), No. 18, an indication of precisely similar 

 peculiarity was obtained on the collodion plate, although at that time 

 sufficient importance was not attached to the difference. 



(7S) 74. Yellow, hy Iron. — The least refrangible rays are scarcely at all 

 influenced by this glass ; the orange is slightly extended upon the red ; and 

 the yellow in a similar manner encroaches on the green rays. The green 

 rays are, however, very decided, and beyond them there still appears an out- 

 standing line of blue, or rather dark indigo ; but beyond these no further 

 rays are visible. 



(74) 74. At the most refrangible edge of the yellow rays the chemical 

 action begins ; and it may be at once described as extending to the very edge 

 of the space occupied by the visible rays of the unabsorbed spectrum. In 

 this, as in the spectrum from the carbon yellow glass (7 1 . 72), 



a like dissimilarity was observable between the action of the S- 37. 



two ends of the chemically active rays, although not to the 

 same extent. The lower space, which commences about 

 •40 above a' and extends to above -60, is a pale gray spot, 

 with a well-defined outline. The upper space, commencing 

 at 'S5 above a' and extending completely up to a, is much 

 broader than the first, less perfectly defined, and of a brown 

 colour of the same character as that already described. 



After several experiments, in which the periods of exposure a_ 



were much varied, it was proved that no chemical action 



took place actually in the yellow rays when this glass was employed. 



(75) 65. Lemon Colour, hy Silver. — This very transparent glass does 

 not exert any peculiar influence on the most luminous rays of the spectrum, 

 beyond giving a peculiar whiteness to the yellow ray ; but produces a very 

 decided effect upon those which are least luminous at the most refrangible 

 end. Beyond the green rays a broad fringe of clear dark blue is still evident, 

 being the whole of the indigo ray, slightly altered in its colour by this ab- 

 sorptive medium ; the violet, however, being entirely wanting. 



f 



