of the Solar Spectrum. 241 



The rays reflected from the metallic mirror of a heliostat 

 entered the dark room through a slit 3 millims. in width. About 

 3 inches from the slit, a crown-glass prism was so placed that the 

 transmitted rays underwent about the minimum deflection. A 

 piece of tinfoil was glued upon the anterior face (that nearest 

 the light) of this prism ; by removing a strip of this foil 3 millims. 

 in width, a second slit was produced immediately before the 

 prism. Of course the prism was properly cleaned from glue at 

 the part whence the tinfoil had been removed. 



The lineal thermo-battery was placed at such a distance from 

 the prism, that the visible spectrum had at that place a width of 

 18 millims. ; this is the same spectral width as in Franz's ex- 

 periments. The thermo-battery was fastened upon the brass rail 

 of Melloni's apparatus, which stood at right angles to the pencil 

 of rays emergent from the prism. The rail, upon which the 

 thermo-battery could be easily moved and fixed, was divided into 

 Paris lines. 



A commutator was introduced in the connexion between the 

 thermo-battery and the multiplier, and the latter was read off" 

 before, during, and after the thermo-battery was subjected to the 

 radiation of any especial family of rays from the prism. 



An example may render this course of observation more clear. 



The thermo-battery was brought into the red of the spectrum ; 

 the solar rays which penetrated through the first slit were then 

 received upon a pasteboard screen ; that is, the thermo-battery 

 was protected from radiation. On closing the commutator, the 

 needle of the multiplier stood at 



-l-3°-5. 

 The screen protecting the prism was now withdrawn, so that the 

 thermo-battery received red rays ; the needle of the multiplier 

 stood at 



-7°. 

 On again intercepting the incident rays, so that the thermo- 

 battery was again protected from radiation, the needle of the 

 multiplier stood again at 



-H3°-5. 



Now the conclusion hence to be drawn is, that the radiation 

 of the red rays upon the thermo-battery had in this instance 

 effected a deflection of 10°- 5. 



The commutator was now reversed, and the observation re- 

 peated in the same manner : it was now found that the radiation 

 of the red rays on the thermo-battery caused a deflection of 9^-5. 

 The mean value, therefore, of tlie thermal action produced by 

 the incidence of the red rays upon the thermo-battery, corre- 

 sponds to a deflection of 10" of the needle (tf the multiplier. 



