78 



Capt. Abney and Col. Festing. [Jan. 15, 



of the thermopile are so feeble that photography would be more 

 serviceable. 



The absorbing media usually used were : — 



1. A solution of potassium dichromate. 



2. Ruby glass combined with orange glass. 



3. Deep orange glass alone (often called "stained red"). 



4. Iodine dissolved in an aqueous solution of potassium iodide. 



5. Iodine in alcohol. 



6. Iodine in carbon disulphide (the violet rays being absorbed by 

 canary glass). 



7. Dyes of different kinds, combined with coloured glasses. 



8. Deep coloured cobalt glass and orange glass. 



Of these the 1st and 3rd were employed when the green rays had 

 not to be cut off ; the 2nd, 4th, and 5th when the whole of the more 

 refrangible end of the spectrum down to the orange had to be absorbed, 

 while the 6th cut off very nearly the whole of the visible spectrum. 

 The dyes employed were soluble in either alcohol or water. 



With No. 8 (deep cobalt and orange glasses) a very long exposure 

 to the dark rays was found necessary to obtain a satisfactory impres- 

 sion on a sensitive plate. 



In these experiments we used the thermopile described in our paper 

 in the " Proc. Roy. Soc." (No. 232, 1884). The source of radiation 

 was an incandescence lamp maintained by a Grove's battery of 40 cells, 

 the filament being approximately at the temperature of 1650°. 



An image of the filament was thrown by a lens on the slit of a 

 collimator, behind which was the dispersing prism. The rays emerging 

 from this were collected by a second lens into a spectrum at a definite 

 distance from the prism, and the thermopile was passed along this 

 spectrum, as described in our paper above mentioned. The absorbing 

 media were placed in front of the slit of the collimator, glass cells 

 one-eighth of an inch of internal thickness being used for containing 

 the liquids. 



The thermogram of the radiation unabsorbed, except by the prism 

 and lenses, was first made (see Curve I, figs. I and II). An empty 

 cell was next placed in front of the slit ; the thermogram (Curve VI, 

 fig. II) shows that no special, but a general absorption has been pro- 

 duced.* A comparison of Curves II, III, and V with Curve I, in 

 fig. I, shows the absorption of radiant energy by orange glass, ruby 

 glass, and cobalt glass respectively. 



Through the kindness of Messrs. Chance Brothers we can state 

 that the orange colour is given by the oxides of silver and antimonv 

 combined, whilst the ruby colour is due to cuprous oxide. The 



* The empty cell -curve has been corrected for reflection from the two inner 

 surfaces of the glass, so that all comparisons are made under similar conditions. 



