144 M. MELLONI ON THE POLARIZATION OF HEAT. 



viz. he varied the distance between the source and the thermo- 

 scope, in order to render nearly constant the quantity of heat 

 radiated upon the instrument. Now it may easily be seen that 

 the variation in the distance of the calorific focus cannot have 

 any injurious influence upon the measures of transmission, be- 

 cause the diathermanous body placed against the opening of 

 the intermediate screen being of very small dimensions, and the 

 sources of heat being always at considerable distances, the most 

 eccentric rays are never deflected more than a few degrees from 

 the perpendicular, which produces no sensible alteration in the 

 quantities of heat reflected and absorbed by the body submitted 

 to radiation, as may be proved by direct experiment, by placing 

 the same calorific source at different distances, and taking each 

 time the transmission of a given lamina, which will be found 

 constant, if everything be well arranged for observations of this 

 nature. But the result is diflferent in experiments of polariza- 

 tion by means of piles ; for the proportion of polarized heat 

 varies generally with the slightest variation of incidence of the 

 calorific rays ; and in the experiments under examination, the 

 alteration in the inclination of these rays upon the piles would 

 necessarily amount to several degrees, considering the proximity 

 of the source to the thermoscope, the extent of the polarizing 

 surfaces, and the absence of any intermediate diaphragm. 



Besides, Mr. Forbes neglected to secure his thermomultiplier 

 from the influence of the heat absorbed by the mica laminae*, 



• A single series of observations will suffice to show the small distance at 

 which Mr. Forbes placed his sources of heat ; and the very appreciable influ- 

 ence of the heating of the piles upon the results. 



Source of heat, copper heated to 400° by an alcohol flame. Distance of the 

 ihermoscopic body, five inches and a half. 



Deviations of the gaWanometer. 

 The plane of refraction of one pile at 0°, the other at 0° 6^° 



(Trans, of the R. S. of Edin. vol. xiii. part i. p. 150.) 

 burgh Philosophical Magazine, vol. vi. p. 212. 



The two piles were placed at the same inclination, in the interior of two gra- 

 duated tubes, turning one within the other; the first was fixed upon the cylin- 

 drical envelope of the thermomultiplier, the second was free, and could move 

 so as to direct the zero of the division successively into the positions indicated 

 by the table. If tho galvanometrical deviations corresponding to each of these 

 positions represented the effect of the radiation transmitted immediately through 

 the piles only, the values of the first, third, and fifth observations would be 

 evidently equal to each other, and the case would be similar with the second 



