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Mr. W. Crookes on the Measurement [Dec. 17, 



I. i( On the Measurement of the Luminous Intensity of Light." By 

 William Crookes, F.R.S. &c. Received June 27, 1868. 



(Abstract.) 



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 satis- 

 factory 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 ascer- 

 tain 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 one great thing to be aimed 

 at is an absolutely uniform source of light. In the ordinary process of pho- 

 tometry the standard used is a candle, defined by Act of Parliament as a 

 " sperm of six to the pound, burning at the rate of 120 grains per hour." 

 This, however, is found to be very variable, and many observers have alto- 

 gether condemned the employment of test-candles as light-measures. 



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

 be at the same time fairly uniform in its results, would not vary by keep- 

 ing, and would be capable of accurate imitation at any time and in any 

 part of the world by mere description. The absence 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 difficulties, the writer believes that he has advauced some 

 distance in the right direction, and pointed out the road for further im- 

 provement. 



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 of sp. gr. 0*805, and 

 pure benzol boiling at 81° C, which are mixed together in the proportion 

 of five volumes of the former and one of the latter. The wick-holder 

 consists of a platinum tube, and the wick is made of fifty-two pieces of 

 platinum wire, each 0*01 inch in diameter. The flame of this lamp forms 

 a perfectly shaped cone, the extremity being sharp, and having no ten- 

 dency to smoke ; without flicker or movements of any kind, it burns when 

 protected from currents of air at a uniform rate of 136 grains per hour. 



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 illuminating power have been 

 observed ; but on placing two of these lamps in opposition, no such varia- 

 tions have been detected. 



