1868. ] Luminous Intensity of Light. 357 
ance so as to keep the galvanometer needle at the same deflection, 
uniform results could be obtained. In practice, however, it was 
found that many things interfered with the uniformity of the results, 
and the light being much feebler than it was advisable to work 
with, this plan was deemed not sufficiently promising, and it was 
abandoned. The method ultimately decided upon is the following :— 
Alcohol of sp. gr. 0°805, and pure benzol boiling at 81 C., are mixed 
together in the proportion of 5 volumes of the former and 1 of the 
latter. This burning fluid can be accurately imitated from descrip- 
tion at any future time and in any country, and if a lamp could be 
devised equally simple and invariable, the ight which it would yield 
would, it is presumed, be invariable. ‘This difficulty the writer has 
attempted to overcome in the following manner. 
A glass lamp is taken of about two ounces capacity, the aperture 
in the neck being 0°25 inch diameter; another aperture at the 
side allows the liquid fuel to be introduced; and by a well-known 
laboratory device, the level of the fluid in the lamp can be kept 
uniform. ‘The wick-holder consists of’ a platinum tube 1°81 inches 
long and 0°125 inch internal diameter. The bottom of this is 
closed with a flat plug of platinum, apertures being left in the 
sides to allow free access of spirit. A small platinum cup 5 inches 
diameter and 1 inch deep is soldered round the outside of the tube 
0-5 inch from the top, answering the threefold purpose of keeping 
the wick-holder at a proper height in the lamp, preventing evapo- 
ration of the liquid, and keeping out dust. The wick consists of 
52 pieces of hard-drawn platinum wire, each 0-01 inch in diameter and 
2 inches long, perfectly straight and tightly pushed down into the 
platinum holder until only 0:1 inch projects above the tube. The 
height of the burning fluid in the lamp must be sufficient to cover 
the bottom of the wick-holder: it answers best to keep it always 
at the uniform distance of 1°75 inches from the top of the platinum 
wick ; a slight variation of level, however, has not been found to 
influence the light to an extent appreciable by our present means 
of photometry. The lamp with reservoir of spirit thus arranged, 
with the platmum wires parallel and their projecting ends level, a 
light is applied, and the flame instantly appears, forming a perfectly- 
shaped cone 1°25 inches in height, the pomt of maximum brilliancy 
being 0°56 inch from the top of the wick. The extremity of the 
flame is perfectly sharp, without any tendency to smoke; without 
flicker or movement of any kind, it burns when protected from 
currents of air at a uniform rate of 136 grains of liquid per hour. 
The temperature should be about 60° F., although moderate varia- 
tions on either side exert no perceptible influence. 
There is no doubt that this flame is very much more uniform 
than that of the sperm candle sold for photometric purposes. 
Tested against a candle, considerable variations in relative illumi- 
