Action of the Rays of the Solar Spectrum. 335 



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

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

 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 Actinograpli, 

 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 intensit}^ 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 therviograph 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- 

 face acquires by the absorption of the liquid into the pores of the 

 paper. He also explains a method by which the impression thus 

 made, and which is only transient, can be rendered permanent. 



This method of observation is then applied to the further exa- 

 mination of various points connected with the distribution of the 

 thermic rays, the transcalescence of particular media, the polari- 

 zation of radiant heat (which is easily rendered sensible by this me- 

 thod), &c. The realit}^ of more or less insulated spots of heat dis- 

 tributed at very nearly equal intervals along the axis of the spec- 

 trum (and of which the origin is probably to be sought in the flint 

 glass prism used' — but 2'>ossibly in atmospheric absorption) is esta- 

 blished. Of these spots, two of an oval form, are situated, the one 

 nearly at, and the other some distance beyond the extreme red end 



