6 Drs Christison and Turner on the Construction of 
illuminating power. Mr Powell lias found, that, before an iron-ball, heated so as 
to be faintly luminous, the thermometric photometer indicated 10 or 13 de- 
grees of light in half a minute, ( Annals of Philosophy , vol. viii. p. 1 88. N. S.) Mr 
Ritchie of Tain has observed that a ball of iron, heated so as to be faintly lu- 
minous in the dark, affected it considerably, ( Edin . Joum . of Science , vol. ii. 
p. 323.) Our own experiments are even more decisive. Before a chamber 
fire in a state of vivid ignition, without flame, the photometer, at the distance 
of 16 inches, fell 25 degrees in one position, and 17-5 inches when turned half 
round. The true effect was therefore 21 \ degrees. Now, at the distance of 
64 inches from a good Argand oil-lamp, its true indication was only 3 degrees. 
Hence, if this instrument was to be relied on to the extent its inventor and 
defenders allege, the fire gave 42 times as much light as the lamp. Neverthe- 
less, according to a rough estimate, founded on the distance at which each of 
us could make out a few words of a book printed in diamond type, and erring 
greatly in favour of the fire, its illuminating power was only a sixteenth part 
of that of the lamp. 
While it is evident, therefore, that the photometer of Mr Leslie is affected 
by non-luminous heat, and that it does not express accurately the illuminating 
power of lights differing in colour, it must at the same time be allowed to give 
indications not very wide of the truth, when their colour and the non-lumi- 
nous heat which accompanies their light are nearly the same. This will ap- 
pear from the following experiment. An oil-gas jet of 4 inches, burning with 
perfect steadiness and uniformity, was placed on one limb of Count Rumford’s 
photometer, at the successive distances of 80, 69|, 5 6| and 40 inches ; so that 
its light on the field of the instrument was in the inverse ratio of 4, 3, 2, and 1. 
On the opposite limb of the instrument, an Argand burner with 20 holes on a 
circle of T e 5 ths of an inch in diameter, was placed at the distance of 1 16| inches ; 
and Mr Leslie’s photometer was carefully fixed four inches from the centre of 
the burner. The gas being kindled, the flame was successively raised till its light 
equalled that of the jet at the distances already mentioned ; and no part of the 
apparatus connected with the burner was moved from the beginning till the end 
of the experiment, except, that, as the flame was successively raised, the bur- 
ner was depressed by weights, so that the photometer was always opposite the 
middle of its flame. The lowest flame was 1 inch, the next 1 J, the next If, and 
the strongest 3 inches. The indications of the photometer, which were never 
finally noted till it was stationary for two or three minutes, were 13, 184 > 27 , 
57|. The true numbers, assuming the first to be correct, and granting that 
Rumford’s photometer, as we shall soon prove, gives true indications, would 
have been 13, 17|, 26, and 52; or, supposing the third correct, 134, 18? 27, 
and 54. The greatest error, therefore, is in the last observation. This pro- 
bably arose from the flame being taller than the glass properly admitted of, so 
that it had a reddish-brown colour at top, and consequently a superior heating 
power at that part. 
Now, independently of what has just been mentioned, these flames differed 
obviously in colour, the lowest being the whitest. Perhaps they differed near- 
ly as much in that respect, as the light of oil- gas differs from that of good coal- 
gas when consumed in proper burners ; and hence the instrument might pro- 
bably be used for the special purpose of ascertaining their relative illumina- 
