Radiant Heat through Crystals to the direction of Transmission. 21 



perpendicular to the optic axes, the quantities transmitted in 

 these respective directions were found to be in the ratio of 

 100 : 139. Although the crystal here examined exhibited cer- 

 tain mechanical defects, M. Knoblauch considers it certain that 

 the differences stated cannot be referred to this cause. In the 

 case of dichroite, radiant heat passes in the direction of the 

 middle line in the greatest quantity ; in less quantity perpen- 

 dicular to the plane of the optic axes, whereas the direction of 

 minimum transmission is that of the supplementary line. 



The qualities of the heat passing in these three directions 

 were next examined. In the first compartment of the following 

 table, the heat transmitted parallel to the middle line is com- 

 pared with that transmitted parallel to the supplementary line; 

 in the second compartment, the middle line and the perpendi- 

 cular to the plane of the optic axes are compared ; and in the 

 third compartment, the supplementary line is compared with 

 the perpendicvdar to the plane of the optic axes. In all cases 

 the quantity faUing tipon the glass is set =100, and the ratio 

 of this quantity to that which passes through ike glass is stated. 



Prom these observations M. Knoblauch concludes, that the 

 properties of calorific rays are different, that is, they possess 

 different powers to pass through the same diathcrmanous body, 

 according as they have traversed dichroite in the direction of 

 the middle line, parallel to the supplementary line, or perpendi- 

 cular to these two directions. 



A cube of blue topaz was submitted to a similar examination : 



