Reflected-!) iff meted and Diffracted-Reflected Rays. 45 



equilibrium as 2*38 X 10~ 5 calorie per hour — we get for the 

 heat production 2 38 x 10" 5 x -56 x 10~ 5 = 1*33 x 10" 10 calorie 

 per gram per hour. The total heat evolved is therefore 

 2 '9 x 10 -10 calorie per gram per hour. 



The small thermal effect due to such low quantities of 

 radioactive matter in rocks, has been pointed out by Professor 

 0. H. Lees (Roy. Soc. Proc. A. vol. Ixxxiii. p. 344), and we 

 should not expect — nor was there found — any abnormally 

 high degree of temperature in the tunnel. 



In conclusion I desire to express my gratitude to Professor 

 Joly, at whose suggestion and under whose directions the 

 work was carried out, and to Professor "W. E. Thrift, for his 

 kindness in providing the use of the room in which the 

 experiments were performed. 



Geological Laboratory, 



Trinity College, Dublin. 



May, 1910. 



III. The Interference of the Reflected-Diffracted and the 

 Diffracted-Reflected Rays of a Plane Transparent Grating. 

 and on an Interferometer. By C. Barus and M. Barus *. 





1. Introductory. 



IF parallel light, falling on the front face of a transparent 

 plane grating, is observed through a telescope after 

 reflexion from a rear parallel face (see rig. 1), the spectrum 

 is frequently found to be intersected by strong vertical inter- 

 ference bands. Almost any type of grating will suffice, 

 including the admirable replicas now available, like those of 

 Mr. Ives. In the latter case one would be inclined to refer 

 the phenomenon to the film and give it no further con- 

 sideration. On closer inspection, however, it appears that 

 the strongest fringes certainly have a different origin and 

 depend essentially on the reflecting face behind the grating. 

 jf for instance this face is blurred by attaching a piece of 

 rough wet paper, or by pasting the face of a prism upon it 

 with water, so as to remove most of the reflected light, the 

 fringes all but disappear. If a metal mirror is forced against 

 the rear glass face, whereby a half wave-length is lost at the 

 mirror but not at the glass face in contact, the fringes are 

 impaired, making a rather interesting experiment. With 

 homogeneous light the fringes of the film itself appear to the 

 naked eye as they are usually very large by comparison. 



* Communicated by the Authors. Abbreviated from a report to the 

 Carnegie Institute of Washington, D.C., U.S.A. 



kd 



