4< Drs Christison and Turner on the Construction of 
I. 
The most essential instrument in experiments of the kind is the Photome- 
ter. Of these, two are well known to the scientific world, that of Professor 
Leslie, and that of Count Rumford. 
As, in making a continuous train of experiments, it was of some moment 
that they should be all susceptible of comparison, and therefore referrible to 
some unvarying standard, it would have been very desirable if we could have 
made use of the Photometer of Professor Leslie. The results he had obtained 
with it, however, differed so much from those procured by all previous ob- 
servers, that it was necessary to ascertain, before confiding in it, whether its 
indications were correct. It is unnecessary to enter now into a full detail of 
our experiments on this head. They led to an investigation of some interest 
regarding certain properties of Radiant Heat ; but as that was foreign to our pri- 
mary object, and other occupations likewise withdrew us from it, we have been 
compelled to leave it unfinished till a future opportunity, and are therefore 
unwilling to enter into particulars at the present moment. It will be suffi- 
cient to state generally the reasons we found for not making use of the Ther- 
mometric Photometer. 
In the first place, it was not delicate enough for our purpose. Some of the 
lights we had to measure did not exceed the fourth part of that of a tallow 
candle, — a quantity which the thermometric photometer could not indicate, 
unless it was either made of such proportions as would render it unfit for 
grosser experiments, or was placed so very near the light, that the slightest 
obliquity in its position must have caused material errors. Secondly , consi- 
dering the vast number of observations we should have to make, this instru- 
ment was ineligible, on account of the long time required for each. In our 
hands, it takes nearly 40 minutes to attain its maximum, and return to its zero. 
But, thirdly , its indications appeared to us fallacious ; and, although subsequent 
observation has led us to alter somewhat the views we formerly entertained 
on this head, yet our experiments, confirmed by others proceeding from much 
higher authority than ours, still bear us out in the opinion, that the thermo- 
metric photometer cannot measure correctly the illuminating power of various 
kinds of lights. 
For , first, it is affected by non-luminous heat. It has been assumed some- 
what hastily that the absorption of non-luminous calorific rays is influenced 
by surface only, and not by colour, — in other words, that differently -coloured 
surfaces will, cceteris paribus , absorb these rays equally well. This doctrine, so 
essential to the principle on which the thermometric photometer is construct- 
ed, is upheld only by a single experiment of Count Rumford’s, which he has 
recorded in his paper on the Communication of Heat in the Philosophical 
Transactions for 1804, and which he himself allows to be unsatisfactory. We 
have made some experiments on this head, which promise results of interest, 
and which we hope soon to lay before the Society. In the mean time, we may 
mention, that whatever may be the fact as to the doctrine now alluded to, 
there can be no doubt that Mr Leslie’s photometer is affected by non-lumi- 
nous heat. 
