1885.] Absorption-spectra Thermograms. 79 



Fig. 1. 



Curve T. Naked incandescence lamp. 

 Curve II. Absorption of orange glass. 

 Curve III. „ ruby 



Curve IV. „ green ,, 



Curve Y. „ dark cobalt glass. 



cobalt colour, as is well known, is given by cobalt. A glance at 

 the curves shows why any combination in which the last is employed 

 would necessitate a prolonged exposure, as it will be seen that it 

 entirely absorbs the infra-red rays about the part of maximum energy. 

 The use of this glass should therefore be avoided, if possible, when 

 the infra-red rays are to be used in photography. 



In a paper by Captain Abney, which appeared in the " Philosophical 

 Magazine " (vol. x, 1880), it was shown that reversal of the photo- 

 graphic image was caused by rays of low refrangibility. If a photo- 

 graphic plate which has been exposed to white light in the camera be 

 subsequently exposed to radiations of low refrangibility, as for instance 

 to light passing through a combination of ruby and orange glass, we 

 should expect that there would be a gradual extinction of the effect 

 of the white light. This is true in practice, though it rarely happens 

 that the after exposure to such radiation is sufficiently prolonged to be 

 hurtful. A case, however, might arise when a knowledge — or perhaps it 



