60 A. M. Mayer — Physical Properties of Vulcanite. 



D is the percentage (32 per cent) of heat effect from a 

 Locatelli lamp transmitted by a plate of obsidian 8 mm thick. 



E is the percentage (16'5 per cent) of heat effect of sun's 

 rajs transmitted by a plate of obsidian 8 mm thick. 



The examination of the curves F and Gr and of the points 

 D and E shows that the rays transmitted by vulcanite and 

 obsidian are of long wave-length ; these are in greater per- 

 centage in the rays from the Locatelli lamp than in those from 

 the sun. A plate of vulcanite one-half millimeter in thick- 

 ness transmits 32 per cent of these rays while of the sun's 

 rays 24 per cent are transmitted. The plate of obsidian of 

 gmm thickness acts in like manner, but it screens even a greater 

 proportion of the sun's rays than it does those from the Loca- 

 telli lamp. 



In making the experiments on the transmission of the sun's 

 rays I used a revolving diaphram pierced with holes of various 

 diameters. The face of this diaphragm was exposed to the 

 solar beam ; sections of which, equal to the area of the dif- 

 ferent holes, were transmitted to the thermopile, which was 

 placed at such distances from the diaphragm that the whole of 

 the rays transmitted were included in the area of the face of 

 the pile. The deflections of galvanometer produced by beams 

 going through holes of different diameters are given below. 



Diameter of hole. Deflection. Diameter of bole. Deflection. 



1) l-25 mm 8*5 3) 4-9 ram 131*5 



2) 3-4 63-0 4) 6-1 204 



If the heat given to pile is as the area of the aperture, then 

 the squares of the diameter of aperture divided by the deflec- 

 tions should equal a constant, as follows : 



1-5625 x 24-01 ,_ 



x ) ^r = - 1838 3 > m^ = - 1826 



2) H^ 6 = -1835 4) ?£" = -1824 



' 63 ; 20-4 



Mean = -18305 



I mention these experiments because they show that such a 

 diaphragm may be of service to those who wish to obtain in 

 succession definite amounts of heat from a solar beam. 



The index of refraction of vulcanite was measured by ascer- 

 taining, in a spectrometer, the angle of maximum polarization 

 of a polished surface of vulcanite. The mean of fourteen 

 measures gave for this angle 57° 29', and the tan. 57° 29 ; = 

 1*568 for the index of refraction ; a number nearly as great as 

 the index of refraction of flint glass. 



Thin lenses of vulcanite and of obsidian were made for me 

 by Mr. John A. Brashear and with them I concentrated the 

 rays of long wave-length to invisible foci on the face of the 

 thermopile. 



Stevens Institute of Technology, Hoboken, N. J. 



