Gas-Burners , and on the Illuminating Power of the Gases. 9 
is gained by that precaution. It is hardly possible, for instance, to compare a 
candle with an Argand’s gas-burner, or a coal-gas jet with an oil-gas burner, 
the differences in colour being so great. When the relation between two such 
lights, therefore, must be determined, a double observation is necessary. 
Thus, in the first example, r the candle must be compared with a gas-jet, 
and then the jet with an Argand’s burner ; and, in the second example, the 
coal-gas jet must be compared with an oil-gas jet, and that with an oil-gas 
burner. In this way we have repeatedly procured very accurate results. Up- 
on the whole, the objection arising from difference in colour is valid only as 
it constitutes an occasional impediment, not as necessarily leading to fallacy. 
The third and last objection is contained in a paper by Mr Hitchie of Tain, 
formerly alluded to, and applies to all the present methods of measuring light. 
Mr Ritchie draws a distinction between quantity of light and illuminating power; 
the former implying the number of particles discharged from a luminous body 
in a given time, the latter the power which these particles have of rendering 
objects visible. The quantity of light, and the illuminating power, he conti- 
nues, are not proportional, except in lights of the same colour ; in the most 
brilliant lights the illuminating power increases in a much greater ratio than 
the quantity of light ; and he maintains, that the photometers of Rumford 
and Leslie, as well as the modification of the latter proposed by himself, are 
defective, because they take cognisance of the quantity only, (Edin. Journ. of 
Science , ii. 324.) In another paper he says, the photometers in question cannot 
be used to ascertain the relative illuminating powers of oil and coal gas, as the 
qualities of their light are essentially different, and the said instruments do 
not take cognisance of the fine white colour of oil-gas, compared with the 
more dusky tint of coal-gas, {Ibid. 341.) On this account, he conceives, that, 
if the photometer indicates the relative quantities of the light of oil and coal 
gas to be as 3 to 1, the real illuminating power, taking into account both quan- 
tity and quality together, may be so high as 5 to 1. These statements, if we 
understand their import correctly, imply, that lights of superior brilliancy, or 
whiteness, or, speaking more precisely, those which, for a given surface, have 
the greatest intensity, besides giving most light, are also possessed of some 
other quality, which renders them fittest for the purposes of vision, and which 
the photometers do not appreciate. 
Mr Ritchie’s views are novel, ingenious, and well deserving of attention, 
but at the same time altogether hypothetical ; nor is it easy to see by what 
facts they can be either substantiated or disproved. In their present state, 
therefore, they are perhaps hardly a fair object of criticism. 
There may be some particular purposes, for which a small intense light is 
better fitted than another equal in quantity, but of inferior intensity. In re- 
gard to the photometer of Count Rumford, however, it may be well to remark, 
that the principle by which the measurement is made, is precisely the principle 
by means of which we take cognisance of most of the properties of external ob- 
jects, that are estimated through the medium of sight. Objects represented 
on a plane surface are distinguished in part by the difference of their colour ; 
but if their colours are simply black and white, as in a printed book, they are 
distinguished in reality by differences'of shade. According as the light in- 
creases, whether in quantity or in intensity, or brilliancy, (to use Mr Ritchie’s 
