412 Einthoven and Jolly 



Had our absolute radiations been exactly determined by the bolometer, 

 it would have been worth our while also to measure f accurately. Since, 

 however, we have based our calculations of the absolute radiation on the 

 ten^rature of the crater, we may content ourselves with an estimation 

 of p. Further, this estimation will suffice, because jp is very small in com- 

 parison with the enormous ratios of the radiating energies employed by us, 

 which have ranged from 1 to more than lO^'*. 



Our estimation of p is based on a measurement which has recently been 

 made by won Kries,^ who gives the analogous loss in his spectroscopic 



arrangement as =1'825. 



According to the above data the amount of the radiation entering the 

 eye is calculated to be as follows : — 



Hi(A = 0-460) = 229 

 Hia = 0-497) = 277 

 Hi(A. = 0-590) = 379 

 Hi(A = 0-670) = 418 



These results are represented -diagrummatically in fig. 25. Here the 

 wave-lengths of the normal spectrum are plotted in. a system of rectangular 

 co-ordinates as abscissae, and the values of H, as ordinatea. 



The radiation h of our three parts of the spectrum expressed in g. cal. 

 per sec. is represented by the area of the three parts of the diagram : — 



For blue, from X = 0460 to X = 0-497 . h^ = 9-36 X 10"* g. cal. per sec. 

 „ green, „ X = 0497 to X = 0-590 . hg = 30-5xl0-* 

 „ red, „ X = 0-590 to X = 0-670 . hr = 31-9xl0-* 



To obtain a maximum of radiation in the eye the largest diaphragm is 

 placed at Dg, viz. the diaphragm for green of 9*5 x 275 mm.* The slit of 

 the collimator and the prism are removed from the spectroscopic apparatus 

 and the uncoloured image of the crater thrown ditectly on the diaphragm 

 Dj. The crater image, which is about 20 mm. high and 32 mm. long, has 

 an elliptic form and overlaps all margins of the diaphragm. 



In order to calculate the radiation energy so obtained, we have to 

 determine the part of the crater to whioh the rectangular diaphragm at 

 Dg corresponds. If this is calculated from the image magnification, a 

 rectangle of 1*9 mm. high and 55 mm. long is found as the piece cut out 

 from the crater. It is by this rectangle, when we employ our maximum 

 amount of white light, that the slit of the collimator is replaced. Its area 

 is exactly five times larger than the area of the slit. 



It is useless to calculate the radiation energy of the crater light for all 

 wave-lengths together. First we must bear in mind that a considerable 

 part of the ultra-red and ultra-violet rays is absorbed by the glass lenses 



• Loc. dt, p. 391. 



