1868. | Luminous Intensity of Light. 355 
It is very likely that a continuance of these experiments would 
lead to the construction of a photometer capable of measuring the 
luminous rays; for although bromide of silver behind quinine is 
not affected by the red or yellow rays, still it is by the green and 
blue ; and as the proportion of red, yellow, green, and blue rays is 
always invariable in white light (or the light would not be white but 
coloured), a method of measuring the intensity of one set of the 
components of white light would give all the information we want; 
just as, in an analysis of a definite chemical compound, the chemist 
is satisfied with an estimation of one or two constituents only, and 
calculates the others. 
Method based upon the foregoing considerations would supply 
us with what may be termed an absolute photometer, the indication 
of which would be always the same for the same amount of ilumi- 
nation, requiring no standard light for comparison; and pending 
the development of experiments which the writer is prosecuting in 
this direction, he has been led to devise a new and, as he believes, a 
valuable form of relative photometer. 
A relative photometer is one in which the observer has only to 
determine the relative illuminating powers of two sources of light, 
one of which is kept as uniform as possible, the other being the 
light whose intensity is to be determined. It is therefore evident 
that the great thing to be aimed at is an absolutely uniform source 
of light. In the ordinary process of photometry the standard used 
is a candle, defined by Act of Parhament as a “sperm candle of six 
to the pound, burning at the rate of 120 grains per hour.” This 
is the standard from which estimates of the value of illuminating 
gas are deduced, hence the terms “12-candle gas,’ “14-candle 
gas,” &c. In his work on Gas Manipulation, Mr. Sugg gives a 
very good account of the difficulties which stand in the way of 
obtaining uniform results with the Act of Parliament candle. A 
true sperm candle is made from a mixture of refined sperm with a 
small proportion of wax, to give it a certain toughness, the pure 
sperm itself being extremely brittle. The wick is of the best cotton, 
made up into three cords and plaited. The number of strands in 
each of the three cords composing the wick of a six-to-the-pound 
candle is seventeen, although Mr. Sugg says there does not appear 
to be any fixed rule, some candles having more and others less, 
according to the quality of the sperm. Sperm candles are made to 
burn at the rate of one inch per hour, and the cup should be clean, 
smooth, and dry. The wick should be curved slightly at the top, 
the red tip just showimg through the flame, and consuming away 
without requiring snuffing. To obtain these results, the tightness 
of the plaiting and size of the wick require careful attention; and 
as the quality of the sperm differs in richness or hardness, so must 
the plaiting and number of strands. A variety of modifying cir- 
