1868. ] ( 353 ) 
VI. ON THE MEASUREMENT OF THE LUMINOUS 
INTENSITY OF LIGHT. 
By Witu1am Croorszs, F.RS., &. 
Tue measurement of the intensity of a ray of light is a problem 
the solution of which has been repeatedly attempted, but with less 
satisfactory results than the endeavours to measure the other radiant 
forces. The problem is susceptible of two divisions :—the absolute 
and the relative measurement of light. 
I. Given a luminous beam, we may require to express its inten- 
sity by some absolute term having reference to a standard obtained 
at some previous time, and capable of being reproduced with accu- 
racy at any time in any part of the globe. Possibly two such 
standards would be necessary, differing greatly in value, so that the 
space between them might be subdivided into a definite number 
of equal parts ; or the same result might perhaps be obtaimed by the 
well-known device of varying the apparent intensity of the standard 
light by increasing and diminishing its distance from the instrument. 
II. The standard of comparison, instead of bemg obtained once 
for all, like the zero and boiling-points of a thermometer, may be 
compared separately at each observation; and the problem then 
becomes somewhat simplified into the determination of the relative 
intensities of two sources of light. 
The absolute method is of course the most desirable; but as the 
preliminary researches and discoveries are yet to be made before 
a photometer, analogous to a thermometer in fixity of standard and 
facility of observation, could be devised, the realization of an abso- 
lute light-measuring method appears somewhat distant. The path 
to be pursued towards the attamment of this desirable object ap- 
pears to be indicated in the observations which from time to time 
have been made by Becquerel, Herschel, Hunt, and others on the 
chemical action of the solar rays, and the production thereby of a 
galvanic current, capable of measurement on a delicate galvano- 
meter, by appropriate arrangements of metallic plates and chemical 
baths connected with the ends of the galvanometer wires. 
Many so-called photometers have been devised, by which the 
chemical action of the rays at the most refrangible end of the spec- 
trum have been measured, and the chemical intensity of light 
tabulated by appropriate methods; and within the last few years 
Professors Bunsen and Roscoe have contrived a perfect chemical 
photometer, based upon the action of the chemical rays of light on 
a gaseous mixture of chlorine and hydrogen, causing them to com- 
bine with formation of hydrochloric acid. 
But the measurement of the chemical action of a beam of light, 
