95 
_Let f represent the film on a section of the glass plate g, perpendicular 
to the diamond scratch s. Let us regard s as a source of radiation. 
All rays (as s ¢) lying outside the critical angle i are totally reflected 
and hence do not affect the film. Those having an incident angle less than 
i penetrate the film and fog it if they are of sufficient intensity. The 
breadth of the fogged line is therefore— 
=2 AB=2 t. tan. i. 
where t is the thickness of the glass plate and i is the critical angle for 
glass and the film substance. 
Taking the indices of refraction of glass and gelatine for violet light, 
it was found that the equation is correct to within the degree of accuracy 
with which the various measurements could be made. 
It was thought that the light produced by the friction of the diamond 
and glass might be sufficient to affect the eye. Nothing could be seen 
when the experiment was tried, although the observers had taken the pre- 
caution of staying in an absolutely dark room for an hour to render the 
eye as Sensitive as possible. But this does not prove that no light resulted 
from the friction. A very feeble light would be sufficient to fog the plate 
when coming from a point so near the film. Besides, the fluorescence 
might have consisted of waves too short to affect the eye. In the formula 
I used the indices of refraction of violet light in order to obtain the value 
of the critical angle. For shorter waves the indices would be different, 
but their ratio probably would not be greatly different from the value 
used. 
Later experiments have shown that fluorescence does not always occur 
when a diamond is drawn across a dry plate. I am not yet ready to say 
whether it is due to differences in different diamonds, to differences in the 
nature of the glass, or to changes in temperature, electrification, ete. I 
hope to be able to report more definitely at a future meeting. 
