18 M. MELLONI ON THE FREE TRANSMISSION 
candle be held for some minutes at a suitable distance, and the com- 
munication then intercepted, the needle will be forced back to zero in 
an interval of time less than 88. These operations would be impossible 
if the side of the pile opposite to the lamp were hermetically closed. 
The second moveable screen serves then to abridge the duration of the 
experiments. It is particularly useful when the calorific action has 
been very powerful or considerably prolonged, which sometimes happens 
in the first attempts at adjustment. During these, the portions of heat 
penetrate the pile toa great depth, and cannot return until a considerable 
time has elapsed. Before these simple means of correction had occurred 
to me, the difficulty of restoring the equilibrium of the two extremes of 
the pile, as well as that which I experienced in respect to ‘he different 
temperatures of the screens and the apparatus, often obliged me to stand 
still for fifteen or twenty minutes between two consecutive experiments. 
When any object of research requires numerous experiments, we 
should endeavour from the very outset to avail ourselves of all that 
contributes to make them more expeditious ; for the least delay arising 
from imperfectness of method will, by gradually accumulating, ulti- 
mately render the labour of whole days utterly fruitless. Yet, the at- 
tention being absorbed by the main object, these little defects are at 
first unnoticed. At length, however, we become sensible of them, and 
endeavour to apply aremedy when it is almost too late. But the result 
of the experiment is not without its use, since it may be more or less 
serviceable in analogous circumstances. This consideration must be 
my apology for the minuteness of detail into which I have entered. 
The first problem that presents: itself, in the series of questions rela- 
tive to the passage of radiant heat through solid bodies, is to determine 
the influence which the degree of their, polish has, and the quantity of 
rays transmitted. In order to solve this, we have but to apply our 
thermometrical method to several screens perfectly similar in all re- 
spects, except as to the state of the surface. 
Out of the glass of a mirror which was very pure, and nine milli- 
metres in thickness, I cut eight pieces sufficiently large to cover the 
central aperture of the screen when they were placed on the stand; and, 
after having removed the quicksilver, I wore them down with sand, 
emery, and other such substances, so as to form by their succession a 
complete series of plane surfaces more or less finely wrought, from the 
first and coarsest to the highest and most perfect polish. These dif- 
ferent pieces reduced to one common thickness of 8™™-371* and ex- 
* All the measures of small degrees of thickness contained in this Memoir | 
have been taken with a pair of calipers with pivots, a species of double com- 
passes, with a spring and with legs of unequal lengths, much used in the manu- 
facture of clockwork. This instrument measures directly, and with great nicety, 
even the fortieth part of a line. 
