452 Imprisonment of Radiation hy Total Reflexion. 



made by Prof. C. E. Mendenhall certainty gives us a satis- 

 factory hypothesis, though there still remain some points 

 which are not quite clear to me. Professor Mendenhall 

 considered that the action of the cleavage planes was merely 

 that they interfered with the imprisonment of radiation by 

 total reflexion. All rays originating within the ball which 

 strike the surface at an angle greater than the critical angle 

 are reflected around the inner surface over and over again, 

 so that they eventually maybe considered as having traversed 

 a layer of the medium of infinite thickness. If the medium 

 has any absorption at all, and no media are perfectly trans- 

 parent, this radiation will be of the same type as that of a 

 black body at the same temperature. Scratches on the 

 surface or internal cleavage planes liberate this energy by 

 interfering with the total reflexion. 



Professor Mendenhall has suggested that I include some 

 of his observations in the present paper. 



He writes me that he made a scratch on the surface of a 

 ball of fused quartz, which was then heated in a Bunsen 

 burner. The black-body temperature of the scratch, as 

 observed with an optical pyrometer, was 850° C, while that 

 of the rest of the ball was barely 600° C. 



From similar considerations he concludes that a uniformly 

 heated, nearly transparent sphere should appear more 

 luminous towards the limb. (A solid incandescent sphere 

 appears as a uniformly illuminated disk, as a result of the 

 cosine law.) I have verified this with a hollow glass bulb 

 filled w T ith a very dilute solution of rhodamine, illuminated 

 by the mercury arc and viewed through red glass. If a spot 

 on the surface of the bulb is finely ground with flour emery, 

 this spot appears extremely luminous when near the limb, 

 as a result of the liberation of the fluorescent rays which 

 are undergoing total internal reflexion. The bulb appears 

 slightly more luminous at the limb, even when the surface is 

 not roughened. I have observed the same thing in films of 

 celluloid stained with rhodamine, which appear much more 

 luminous when viewed obliquely, at nearly the angle of 

 grazing emergence, than when observed by rays which leave 

 the surface nearly parallel to the normal. 



In regard to the microcosmic bead, I am now of the opinion 

 that most of the light comes from the hot wire. It is difficult 

 to devise a means of supporting a fused bead on a non- 

 luminous support ; but I have watched their fall from the fifth 

 floor of the laboratory at night, and have found no evidence 

 of any brightening during the drop of five stories. 



