20 Prof. Knoblauch on the Relation of the Transmission of 



manous substance, may be increased from zero to a very great 

 amount, by simply augmenting the thickness of the crystal. 



Similar experiments were made with red, blue and yellow 

 glass. The results are contained in the following table : — 



From this table we derive the general conclusion, that the 

 qualitative difference of the rays which we have here compared 

 become more evident as the thickness of the crystal through 

 which they pass is increased. 



In all the experiments hitherto recorded, the direction of the 

 calorific rays was that of the line which bisected the acute angle 

 enclosed by the two optic axes. M. Knoblauch next proceeded 

 to compare the quantities of heat transmitted in different direc- 

 tions by the same crystal. 



The Nicholas prism was removed, and the direct solar rays 

 were permitted to fall upon a cube of dichroite ; the rays first 

 passed in the direction of the middle line just referred to, and 

 afterwards in the direction of the line which bisected the obtuse 

 angle enclosed by the optic axes, the " supplementary line." 

 The quantity transmitted in the former case, compared to that 

 transmitted in the latter, was as 100 : 36. 



The crystal was next placed so that the supplementary line, 

 and with it the plane of the optic axes, was vertical, and the 

 quantity of heat transmitted along the middle line was com- 

 pared with the quantity transmitted in a direction at right 

 angles to the plane of the optic axes : these quantities were 

 found to be in the ratio of 100 : 46. 



In both these cases, therefore, the quantity transmitted along 

 the middle line was greatest. 



Comparing the direction of the supplementary line with the 



