46 M. ^fELLONI ON THE IMMEDIATE TRANSMISSION 



Before we proceed to consider these results, it is necessary to recollect 

 that they have all been obtained under the free action of an invariable 

 radiation of 30° measured by the thermomultiplier. Now the half de- 

 grees of the galvanometer are very distinctly legible. Thus the trans- 

 missions are exact to ^Vth of the incident heat ; but the observations 

 being repeated, the hundredth part becomes easily appretiable. 



In the quantity of rays transmitted through the same substance there 

 is a vai-iation of several hundredth parts according to its greater or less 

 purity. It was therefore useless in giving the measure of this element 

 to attempt a degree of exactness exceeding the hundredth part of the 

 whole ; but it was desirable to ascertain the limits of the insensible trans- 

 missions with more precision. In this case therefore I have always car- 

 ried the approximation to -.t.Vtt, and sometimes to tJt, so that if the zero 

 does not represent a transmission really equal to nothing, it is at least 

 certain that, if there are any rays of heat transmitted, their amount does 

 not exceed ^A^odth of the whole incident quantity. 



In order therefore to reduce the probability of error, it has been found 

 necessary to operate on stronger radiations. Now the table of intensi- 

 ties given in my first Memoir does not exhibit the forces which move the 

 galvanometric index beyond the 45th degree. I could have extended 

 it to the higher degrees of the quadrant by the method followed in its 

 construction. But I thought it better to employ at each step a very 

 simple artifice which immediately gives the force of any radiation what- 

 soever as well as tlie required limit of error. To make tliis clear, let us 

 suppose that it is desired to verify a particular case of the transmissions 

 in the table ; for instance, that it is requisite to prove that the transmis- 

 sions of alum, sugar, or ice exposed to the rays emitted by copper heated 

 to 390" are either null or less than Trrcdth of the whole of the incident 

 heat. 



The table shows that a plate of glass, of rock ciystal, or of Iceland 

 spar transmits from five to six hundredths of those rays; that is. to 

 say, that for a free radiation of 30° we obtain about 2° through the 

 plate. We know moreover that in this feeble indication there is a pos- 

 sible error of Ath of the whole heat. The limit of error would be ^^ 

 if we wished to be rigorously exact, for by the table of intensities we 

 see that, in the deviations below 20°, one degree is equivalent to -^ of 

 the force which moves the needle to 30°. But let us admit only the 

 limit A) which will have the advantage of rendering the values inde- 

 pendent of a knowledge of the ratios existing between the degrees of 

 the galvanometer and the corresponding forces of deviation. Let us 

 bring the source near, in order that We may obtain through the same 

 plate of glass a deviation exceeding 2° ; a deviation, for instance, of 8". 

 The quantity of incident heat is now increased fourfold, and the pro- 



