of the Prismatic Spectrum. 35 1 



more remote spot I liavo hardly a doubt ; but it is very difficult 

 to obtain a sight of it." 



Another method, also discovered by Herschcl, depends upon 

 the use of papers prepared with red prussiate of potash and 

 perchloride of iron. The mixed solution (which, if the salts 

 are pure, is at first free from green or blue precipitate) is washed 

 over paper under the influence of the spectrum. The thermal 

 rays manifest themselves by a brown coloration, which after- 

 wards changes to blue. The action of the blue and violet rays 

 is purely photographic, and produces at once the deposit of 

 Prussian blue ; but the impression of the ultra-red appears to 

 be due rather to thermometric heat. In this way Herschel 

 obtained a representation of the thermal spectrum " as far as 

 S, with even some trace of e" *. 



Notwithstanding this concurrence of evidence, there is great 

 difficulty in accepting Herschel's result. The heat-spots h and 

 € are decidedly beyond the position at which dispersion should 

 cease; and it is doubtful whether there should be thermal effect 

 even so far down as y. I have examined one of Herschel's 

 spectraf in which the positions of the fixed lines are indicated, 

 and find for the limit of refrangibility a position close to the 

 spot 7 ; but the data are hardly precise enough for more than 

 a rough estimate. 



Wishing, if possible, to find the explanation of the discre- 

 pancy, I have made many attempts to repeat Herschel's expe- 

 riments, but without obtaining any evidence of thermal action 

 nearly so low down in the spectrum as that recorded by him. 

 The sun's rays were reflected horizontally by a silver-on-glass 

 mirror, or by a piece of common looking-glass, into a darkened 

 room, where they fell upon the prism. In order to obtain a 

 spectrum of about the same purity as Herschel's, I generally 

 employed a bisulphide-of-carbon prism of 60°, giving a beam 

 containing about three square inches of light. The prism was 

 placed in such a position as to give minimum deviation for the 

 extreme red. To collect the rays various lenses were tried, in- 

 cluding a single lens of 24 inches focus and a photographic por- 

 trait-lens of about 8 inches equivalent focus, the latter of which 

 gave an exceedingly intense spectrum. In some experiments 

 a heliostat was employed; and in others the solar image was 

 kept nearly in one place by hand. In order to guide the eye, 

 the waste light reflected from the first face of the prism may 

 be collected to a focus by a suitable lens and thrown upon a 

 small hole in a piece of card. Even a small apparent motion 

 of the sun will then give rise to a reappearance of light at the 



* PMl. Mag. vol. xxii. (1843). 



t Photographic News, September 9, 1859. 



