372 
MESSKS. W. E. WILSOX AXD P. L. GEAY 
The question may arise as to whether it is correct to consider the reflection from 
the front surface of the heliostat mirror only, or whether multiple reflections from the 
back surface might not appreciably increase the total amount of heat reaching the 
radio-micrometer. That the former idea is coiTect will be evident from the followino- 
O 
considerations :— 
The glass of the mirror was sufficiently thick to clearly separate (at the angles of 
incidence ordinarily used in our experiments), the image given by the first ordinary 
reflection from the first given after a “back-reflection,” supposing such to exist. 
We focussed a telescope on the image of the sun in the mirror, but could not discover 
even a faint ghost of a second image, thus showing that, at least for all wave-lengths 
