1882.] on Spectrum Analysis in the infra red of the Spectrum. 59 



Some eiglit years ago I tried my hand at the matter, and after 

 several years of exj)erimenting it was my good fortune to find a com- 

 pound which was chemically acted upon by the dark radiations. 



I will not weary you with the various experiments undertaken ; 

 suffice it to say that silver bromide was selected as the salt to work 

 upon. My aim was to prepare an emulsion of bromide of silver in 

 collodion (an emulsion being silver bromide in a fine state of 

 division suspended in collodion) which should transmit green-blue 

 light. Let me show you why. 



The sjDcctrum on the screen, when unabsorbed by any medium, 

 shows every colour. If, however, a j)iece of green glass is inserted 

 before the slit, it is seen at once that the violet is absorbed and also the 

 lowest jDart of the red. Inferentially it may be supposed that the 

 infra-red rays are also absorbed. Orange is the usual colour of the 

 silver bromide, and a piece of orange glass placed in front of the 

 slit cuts off from the spectrum all the most refrangible part of 

 the spectrum and none of the least refrangible. 



On the principle of conservation of energy, where radiation is 

 absorbed, there work must be done by the absorbing body and show 

 itself as heat or chemical action. Heat with the thermopile, for 

 instance, and chemical action ^T.th the salt of silver. Thus, with the 

 orange bromide we should expect, as we have already seen is the 

 case, that the violet and blue rays w^ould do work on it whilst the 

 other rays would be passive. 



The green state was attained after much labour. The colour of 

 this new preparation of silver bromide, and that of the old, are now 

 shown by means of the lantern on the screen. 



Now you will see that if the work done in the green bromide 

 was chemical decomposition, the problem was solved, and that the 

 unknown might be made to write down, in hieroglyphics perhaps, 

 but still in a manner capable of being deciphered, its character and 

 peculiarities. 



I will endeavour to experimentally illustrate that the green 

 compound is acted upon by the dark rays. It will be in your recol- 

 lection that Professor Graham Bell's recently introduced photophone 

 is in reality an instrument consisting of a perforated disc rotating in 

 front of a source of radiation, and by means of a lens the radiant 

 energy is focussed on the surface of a selenium cell, to which a 

 telephone is attached, and that by this means a musical sound is pro- 

 duced in the telephone. 



Professor Bell showed that the same effect was produced when a 

 piece of ebonite was introduced between the source of light and the 

 selenium cell. Dr. Huggins proposed to me that I should try the 

 permeability of the ebonite by the dark rays ; and this was done, with 

 the result that the spectrum was taken through it, showing an im- 

 pression on the green bromide of the dark rays. An image of the 

 incandescent carbon points of the electric light are now formed on a 

 piece of ebonite, and behind it is a glass plate covered with the 



