84 L. Schwendler—On a new Standard of Light. [No. 2, 
Old Standards for Light Measurements.—Up to the present in England 
the Standard Candie* has been adopted as the standard of light, the 
unit of light being defined as that light which the said candle emits 
when burning steadily at a certain definite rate. In France the Oarcel 
Burner (Bee Carcel) has been introduced as the standard of light. 
The unit of light in this case being defined as that light which emanates 
from a good moderator lamp burning pure colza oil, at a given definite 
rate. The ratio of these two arbitrary units, is given by several authorities 
very differently, the mean value being about :— 
10 Standard Candles = 1 Carcel Burner. 
These two standards of light, although answering perhaps certain 
practical requirements, are by their nature ill-adapted to form the units of 
light intensities. A good and trustworthy standard should possess absolute 
constancy, or if not, should afford the possibility of application of a correc- 
tion for the variation and, moreover, should be capable of accurate repro- 
duction. These qualifications are certainly not possessed by the standards 
at present in use. 
A candle of whatever compound and size will partake of something of 
the nature of a complex body, an accurate reproduction of which must 
always be a matter of great difficulty. Exactly the same holds good for the 
Carcel Burner. 
Further the amount of light these standards produce, depends to a 
very considerable extent on external influences, which do not allow of easy 
control or measurement, and which therefore cause variations in the stan- 
dard light for which it becomes impossible to introduce a correction. For 
instance, the rate and regularity with which a candle burns and the amount 
of light it gives, depend, in addition to the material of which the candle 
consists, on the ready and regular access of oxygen. In a closed up place, 
like the box of a photometer, if the draught is not well regulated or the 
supply of fresh air not quite constant, it can be easily observed that the 
very same candle may emit light at different times varying as much as 
50 percent. Another difficulty is introduced by the variation of the length 
of the wick, and of the candle itself, by which the standard light necessarily 
alters its position in the photometer and consequently its quantitative 
* The Metropolitan Gas Act 1860 (23 and 24 Vict. Cap. 125, Sec. XXV) defines 
the standard candle as :— 
“Sperm candles of 6 to the pound each burning 120 grains an hour.” I have tried 
the standard candles as made by two different manufacturers, Messrs. Field and Co. and 
Mr. Sugg. These candles are sold as six to the pound, and consume according to my 
own experiments about 8.26 Gm per hour when placed ina large room and direct 
draughts excluded, 
