﻿Royal Society. 227 



The romance of algebra presents few episodes more wonderful 

 than this specimen of the way in which the determination of the 

 degree of an equation resulting from elimination can be made to 

 contribute a new and by no means obvious fact to the Calculus 

 of Differences. 



Athenaeum Club, 

 February 23, 1869. 



XXXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 145.] 



December 17, 1868. — Captain Richards, R.N., Vice-President, in 

 the Chair. 



THE following communication was read : — 

 "On the Measurement of the Luminous Intensity of Light." 

 By William Crookes, F.R.S. &c. 



The measurement of the luminous 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. 



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

 ascertain the relative illuminating powers of tw T o 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 one 

 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, 

 denned by Act of Parliament as a " sperm of six to the pound, burn- 

 ing at the rate of 120 grains per hour." This, however, is found to 

 be very variable, and many observers have altogether condemned the 

 employment of test-candles as light-measures. 



The author has taken some pains to devise a source of light which 

 should be at the same time fairly uniform in its results, would not 

 vary by keeping, and would be capable of accurate imitation at any 

 time and in any part of the world by mere description. The ab- 

 sence of these conditions seems to be one of the greatest objections 

 to the sperm-candle. It would be impossible for an observer on 

 the continent, ten or twenty years hence, from a written description 

 of the sperm-candle now in use, to make a standard which would 

 bring his photometric results into relation with those obtained here. 

 Without presuming to say that he has satisfactorily solved all diffi- 

 culties, the writer believes that he has advanced some distance in the 

 right direction, and pointed out the road for further improvement. 



A glass lamp is taken of about 2 ounces capacity, the aperture 

 in the neck being 0*25 inch in diameter; another aperture at the 

 side allows the liquid fuel to be introduced ; this consists of alcohol 



Q2 



