100 KNOBLAUCH ON THE TRANSMISSION 



Under these circumstances, it was observed that the calorific 

 rays, when transmitted parallel to the axis of the crystal, pro- 

 duced a deflection of the needle of the multiplying galvanometer 

 amounting to 34°-05*. When, however, the rays were trans- 

 mitted in a line perpendicular to the axis of the crystal (the 

 latter having been turned 90° round), the deflection produced 

 was only 31°'19. 



The ratio of these numbers, which, as in most of the cases 

 hereafter cited, are the arithmetic means of twelve different ob- 

 servations, is as 100 : 92. 



The quantity of heat, therefore, transmitted by a brown rock- 

 crystal, in a line perpendicular to its axis, is less than the quan- 

 tity transmitted in a direction parallel to the same. 



There is no reason whatever to suppose that this difference is 

 in any way attributable to accidental imperfections on the part 

 of the crystal. 



The same experiment was made with a cube of beryl', here 

 also a weaker power of transmission was exhibited perpendicular 

 to the axis than parallel to the same. 



If, as in the former case, we express by 100 the quantity of 

 heat transmitted parallel to the axis of the beryl, then the 

 quantity transmitted perpendicular to the axis, as measured by 

 the multiplier, will be expressed by the number 54. This result 



* When accurate determinations with the thermo-electric pile are required, 

 it is not sufficient to simply read off the deflection ; for this is not due to the 

 temperature which is to be measured alone, but also to accidental differences 

 of temperature between both faces of the pile. The amount contributed by the 

 latter source to the total effect must therefore be ascertained ; a metallic screen 

 is placed in the path of the rays which totally intercepts them, without altering 

 the accidental difference spoken of. The deviation of the needle which remains, 

 indicates the true zero, from which we must reckon in the subsequent deter- 

 mination. If, for example, the total deflection were 35°05, and the needle, 

 after the introduction of the metallic screen, remained standing at 1°, the action 

 to be attributed to the incident rays would amount to .34°'0o. 



All the values contained in the present memoir have been determined in this 

 way. Each of the numbers whose arithmetic mean we have taken is thus the 

 result of two readings of the thermo-multiplier, one of which was simply to 

 ascertain the true zero for the observation made at the time. 



The deflections of the needle are to be considered as the measure of the 

 quantity of heat as within the limit of 35° (according to a special examination 

 of the instrument); the deflections were proportional to the forces which pro- 

 duce them. 



