Imprisonment of Radiation by Total Reflexion. 451 



is represented by 



1 — cos 6 

 cos 6 ' 



which is the ratio o£ the area on a sphere described around 

 the luminous molecule, cut out by the cone of rays EXF, 

 to the area of the hemisphere minus this portion. For celluloid 

 the critical angle is about 40°, i. e,, = 40° and 



1 — cos# A 01 



COS0 



in other words, the imprisoned energy is about three times 

 greater than the energy which escapes. Its liberation by 

 the matt surface should therefore give us about fourfold 

 increase in luminosity. This point was tested with the 

 photometer. As a standard source a rhodamine screen 

 (matt) illuminated by the light of the mercury arc was used, 

 its light being passed through two Nicol prisms by which its 

 intensity could be varied by a known amount. It was 

 compared with a rhodamine film in part covering white 

 porcelain , and in part highly polished silver. The photo- 

 meter used was a very simple but very satisfactory one made 

 by Dr. Pfund, a semicircle of silvered glass taking the place 

 of a Lummer-Brodhun prism. Equality of luminosity of 

 the two fields of the instrument, with disappearance of the 

 dividing line, was secured in the two cases with the nicols 

 rotated 11° and 22° from the position of extinction. Which 

 means that the rhodamine film on porcelain is 3*8 times 

 brighter than the one on silver, which is in good agreement 

 with the calculated value (4). In making the observations 

 a red glass was placed in front of the photometer to exclude 

 everything except the fluorescent light. (White porcelain 

 alone, illuminated by the mercury arc, appeared practically 

 black when viewed through the red glass.) On completing 

 the work it occurred to me that the case was similar to the 

 one to which I drew attention several years ago, namely that 

 a fused bead of microcosmic salt, raised to the highest tempera- 

 ture possible with a blast-] amp, emits little or no light when 

 removed from the flame, though the platinum wire supporting- 

 it glows vividly. This is of course a fine case of the failure 

 of a transparent body to radiate, which is what KirchhofPs 

 law demands. But as the bead cools down it solidifies and 

 numerous cleavage planes appear. At the moment of 

 crystallization it glows with a dull red light, which persists 

 for several seconds. I found difficulty in explaining this 

 (see Phys„ Optics, 2nd edition, page 598), but a suggestion 



2 12 ' 



