M. MELLONI ON THE REFLECTION OF RADIANT HEAT. 385 
whence we derive 
R =1 + 0/0923 = 1 + 0°9607. 
As the first sign of the radical would lead to an absurd result in the 
case under consideration, it must be rejected. The reflection at the 
surface of the plate will then be 1 —-09607= 0°0393 to the unit of inci- 
dence, and such also will be the ratio of the second reflection to the quan- 
tity of heat which reaches the further surface of the rock salt. But if 
it be desired to have the absolute value of the second reflection, it will 
be found by substituting 0-0393 for R in the expression R (1—R), or 
yet more simply by taking the difference between the numbers 0:077 
and 0:0393, which gives in each case 0°0377. 
It now remains to be seen whether the quantities of heat reflected by 
other transparent substances be equal or not equal to those produced by 
the surfaces of rock salt. For the solution of this question it is suf- 
ficient to observe, that a thick plate of glass, of rock crystal, or any other 
diaphanous substance gives a calorific transmission sensibly equal to that 
of another plate, the same in substance though different in thickness. If 
we take, for instance, one plate of glass 8 millimetres * thick, and an- 
other 82, and expose them separately to the radiation of a Locatelli 
lamp, we shall find no sensible difference between the two quantities of 
heat transmitted. From this experiment it is obvious that the layer of 
half a millimetre, which constitutes the difference of thickness between 
the two plates, causes no appreciable absorption of the calorific rays, 
which have already traversed 8 millimetres of the same substance. Let 
us now detach this thin layer from the thicker plate and expose it (thus 
separated) to the rays emerging from the plate of 8 millimetres; it will 
reflect part of them and transmit all the rest. The quantity lost will 
therefore represent the effect, and only the effect, of the two reflections. If 
this experiment be made with care, the number found as the quantity 
transmitted will be very nearly 0°923+, and therefore the number repre- 
senting the part reflected will still be 0077. This is true not only with 
mingle with the bundle of calorific rays directly transmitted, but will be thrown 
laterally. If their action on the thermcmultiplier be appreciable, it must be 
perceived in the diminution of the effect. But the galvanometer always ex- 
hibits the same deviation, both when the plate stands perpendicularly and when 
it stands obliquely. These multiplied reflections have, therefore, no sensible 
influence whatsoever on the measure of the transmission which is represented 
exactly by the number 0°923. 
* 0-315 in. c 
+ Are these small variations, which do not amount even to hundredth parts, 
to be attributed to a difference of polish in the surfaces, or to a difference of 
energy in their reflecting powers? This question seems not to admit of an 
easy solution by experiment. However, if we were to judge by the eomplete 
analogy which these phenomena bear to those that occur in the reflection of 
light, we should think it highly probable that the trifling differences observed 
partly depend on the indices of refraction in the different substances of which 
the plates are composed. 
