Transmission of Infra-red Rays and of Radiant Energy. 75 



way. The depth of the silver deposit was then measured in the areas 

 corresponding to the glasses. The densities corresponding to the step-wedge 

 were measured in a similar manner, and the results plotted against the 

 known values of the wedge absorption. The densities in the areas corre- 

 sponding to the glasses were then referred to this curve, and the absorption 

 stated in percentage of the incident radiation. 



The infra-red radiation at different wave-lengths was obtained by the infra- 

 red spectrophotometer used in testing the absorptions of the eye. 



The results obtained by us are given in Tables VII and VIII, pp. 73-74. 

 Comparison of the values for the different glasses shows the great infra-red 

 absorbing power of the Crookes glasses compared with the other glasses tested. 

 The transparency of these glasses is very considerable, while their ultra-violet 

 absorbing power is not so great as some of the more heavily coloured orange 

 and green glasses previously manufactured. 



The formulae of the Crookes glasses were approximately as follows : — 



No. 256* 



Per cent. 



Soda flux 81 



Cerium nitrate 11 



Ferrous oxalate 54 



Tartar 2 



Charcoal 0*5 



No. 246.f 



Soda flux 90 



Ferrous oxalate 10 



Tartar — 



Charcoal — 



No. 217-f 



Soda flux 96-8 



Ferrosoferric oxide 2*85 



Carbon 0"35 



To facilitate comparison between the optical properties of glasses made to 

 the above formulse we have calculated from the values in Table VII the 

 thickness of plate required to reduce the visual rays by 50 per cent. From 



* Sir W. Crookes kindly sent us the formula of this glass for publication, 

 t 'Phil. Trans.,' A, vol. 214, p. 20 (1914). 



G 2 



