209 



12. The Analysis of the Chemical Rays of the Spectrum by ab- 

 sorbent media, which forms the subject of the next section, opens a 

 singularly wide field of inquiry ; and the author describes a variety 

 of remarkable phenomena which have presented themselves in the 

 course of his experiments on this subject. They prove that the pho- 

 tographic properties of coloured media do not conform to their co- 

 lorific character ; the laws of their absorptive action as exerted on the 

 chemical, being different and independent of those on the luminous 

 rays : instances are given of the absence of any darkening effect in 

 green and other rays of the more refrangible kind, which yet produce 

 considerable illumination on the paper that receives them. 



13. The exalting and depressing power exercised by certain media, 

 under peculiar circumstances of solar light, on the intensity of its che- 

 mical action. This branch of the inquiry was suggested by the fact, 

 noticed by the author in his former communication, that the dark- 

 ening power of the solar rays was considerably increased by the in- 

 terposition of a plate of glass in close contact with the photographic 

 paper. The influence of various other media, superposed on pre- 

 pared paper, was ascertained by experiment, and the results are re- 

 corded in a tabular form. 



14. The paper concludes with the description of an Actinograph, 

 or self-registering Photometer for meteorological purposes ; its ob- 

 jects being to obtain a permanent and self-comparable register and 

 measure, first, of the momentary amount of general illumination in 

 the visible hemisphere, which constitutes day-light ; and secondly, 

 of the intensity, duration, and interruption of actual sunshine, or, 

 when the sun is not visible, of that point in the clouded sky behind 

 which the sun is situated. 



In a postscript, dated March 3rd, 1840, the author states that he 

 has discovered a process by which the calorific rays in the solar spec- 

 trum are made to affect a surface properly prepared for that pur- 

 pose, so as to form what may be called a thermograph of the spec- 

 trum ; in which the intensity of the thermic ray of any given refran- 

 gibility is indicated by the degree of whiteness produced on a black 

 ground, by the action of the ray at the points where it is received 

 at that surface, the most remarkable result of which is the insula- 

 tion of heat-spots or thermic images of the sun quite apart from the 

 great body of the thermic spectrum. Thus the whole extent over 

 which prismatic dispersion scatters the sun's rays, including the 

 calorific effect of the least, and the chemical agency of the most re- 

 frangible, is considerably more than twice as great as the Newtonian 

 coloured spectrum. 



In a second note, communicated March 12, 1840, the author de- 

 scribes his process for rendering visible the thermic spectrum, which 

 consists in smoking one side of very thin white paper till it is com- 

 pletely blackened, exposing the white surface to the spectrum and 

 washing it over with alcohol. The thermic rays, by drying the 

 points on which they impinge more rapidly than the rest of the sur- 

 face, trace out their extent and the law of their distribution by a 

 whiteness so induced on the general blackness which the whole sur- 



