360 Mr. W. Crookes on the Measurement of the 



to one hundred parts of dilute sulphuric acid ; this was placed across the 

 path of the ray of light, and photographs of the spectrum were again taken 

 on iodide of silver and on bromide of silver, the arrangements being, in all 

 cases, identical with those in the first- cited experiments, with the exception 

 of the interposition of the quinine screen. The action of the sulphate of 

 quinine upon a ray of light is peculiar ; to the eye it scarcely appears to 

 have any action at all, but it is absolutely opaque to the ultra-violet, so- 

 called chemical rays, and thus limits the photographic action on the bro- 

 mide and iodide of silver to the purely luminous rays. On developing the 

 latent images, it was now found that the action on iodide of silver was con- 

 fined to a very narrow line of rays, close to the fixed line G, and in the 

 case of bromide of silver, to the space between b and G. Designating the 

 spaces of action by colours instead of fixed lines, it was thus proved that, 

 behind a screen of sulphate of quinine, iodide of silver was affected only by 

 the luminous rays about the centre of the indigo portion of the spectrum, 

 whilst bromide of silver was affected by the green, blue, and some of the 

 indigo rays. 



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 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 calcu- 

 lates the others. 



Methods 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 illumination, requiring no 

 standard light for comparison ; and pending the development of experi- 

 ments 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 photo- 

 meter. 



A relative photometer is one in which the observer has only to deter- 

 mine 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 pro- 

 cess of photometry the standard used is a candle, defined by Act of Par- 

 liament 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 f Gas Manipulation,' Mr. Sugg 

 gives a very good account of the difficulties which stand in the way of 



