64 Sir John Conroy. On the Amount of Luminous and 



The agreement between the values of p, as deduced from the 

 measurements made with each of the four cells, confirms the general 

 accuracy of the photometric observations. 



Glass and water transmit, as is well known, radiations differing in 

 wave-length with, very different degrees of facility ; all the kinds of 

 radiations which affect the eye as light suffer about the same amount 

 of absorption (i.e., these media are colourless) ; but, as Melloni 

 showed many years ago, the case is very different when the total 

 radiation from any source of light and heat is considered. 



Table VIII gives the percentage amounts of total and visible radia- 

 tion transmitted by the four cells, and also the transmission co- 

 efficients A and a. for the total and visible radiations, as deduced from 

 the measurements made with each pair of cells. 



Table VIII. 





Per cent. 



Per cent. 









total 



visible 



A. 



a. 





radiation 



radiation 





transmitted. 



transmitted. 







Cell I. 











3 ram. glass and 12 mm. water 



4-194 



86 -92 . 







Cell II. 







-8529 



0-9977 



3 mm. glass and 52 mm. water 



2-219 



86 -14 1 







Cell III. 







0-9515 



0-9975 



3 mm. glass and 102 mm. water 



1-731 



85-08 | 







Cell IV. 







-9900 



0-9990 



3 mm. glass and 152 mm . water 



1-646 



84 -68 J 







The table shows that the percentage amount of visible radiation 

 absorbed by the water increases regularly with the thickness, but that 

 in the case of the total radiation each additional centimetre of water 

 absorbs less than those that have preceded it, and that the trans- 

 mission coefficients for the total radiation increase as the thickness 

 increases, whilst those for the visible radiation remain nearly constant. 

 From the values of these coefficients it appears that a thickness of 

 3 mm. of glass and 102 mm, of water is not sufficient to arrest all 

 the non-luminous radiations emitted by an Argand gas burner. The 

 transmission coefficients for the total and visible radiations as deduced 

 from the measurements made with Cells III and IV are much closer 

 together than those deduced from the measurements made with 

 Cells I and II, and II and III, and this seems to show that the 

 amount of non-luminous radiation which passed through Cell IV was 

 very small, and that, therefore, the 1"646 per cent, transmitted con- 

 sisted almost exclusively of visible radiation, i.e., light. 



The photometric measurements show that 84'68 per cent, of the 



