46 M. MELLONI 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 ¢oth 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 variation 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 x5», and sometimes to z}s, 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 z4odth 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. T 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 the required limit of error. To make this 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 zs0dth of the whole of the incident 
heat. = 
The table shows that a plate of glass, of rock erystal, 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 oth of the whole heat. The limit of error would be an 
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 aaaiof 
the force which moves the needle to 30°. But let us admit biify the 
limit z's, 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- 
