Luminous Intensity of Light. 



363 



liancy being 0*56 inch from the top of the wick. The extremity of the 

 flame is perfectly sharp without any tendency to smoke ; without flicker 

 or movement of any kind, it burns when protected from currents of air 

 at a uniform rate of 136 grains of liquid per hour. The temperature 

 should be about 60° F., although moderate variations on either side exert 

 no perceptible influence. Bearing in mind Dr. Frankland's observations 

 on the direct increase in the light of a candle with the atmospheric pres- 

 sure, accurate observations ought to be taken only at one height of the 

 barometer. To avoid the inconvenience and delay which this would occa- 

 sion, a table of corrections should be constructed for each O'l inch variation 

 of barometric pressure. 



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 vari- 

 ations have been detected. The same candles have been used, and the 

 experiments have been repeated at wide intervals, using all customary pre- 

 cautions to ensure uniformity. The results are thus shown to be due to 

 variations in the candle, and not in the lamp. 



It is expected that whoever may be inclined to adopt the kind of lamp 

 here suggested will find not only that its uniformity may be relied upon, 

 but that, by following accurately the description and dimensions here laid 

 down, each observer will possess a lamp of equivalent and convertible 

 photometric value ; so that results may not only be strictly comparable 

 between themselves, but, within slight limits of accuracy, comparable with 

 those obtained by other experimentalists. The dimensions of wick &c. here 

 laid down are not intended to fix the standard. Persons engaged in pho- 

 tometry as an important branch of their regular occupation will be better 

 able to fix these data than the writer, by whom photometry is only occa- 

 sionally pursued as a means of scientific research. Already many improve- 

 ments suggest themselves, and several causes of variation in the light have 

 been noticed. Future experiments may point out how these sources of 

 error are to be overcome ; but at present there is no necessity to refine our 

 source of standard light to a greater degree of accuracy than the photo- 

 metric instrument admits of. 



The instrument for measuring the relative intensities of the standard 

 and other lights next demands attention. The contrivances in ordinary 

 use are well known. Most of them depend on the law in optics, that the 

 amount of light which falls upon a given surface varies inversely with the 

 square of the distance between the source of light and the object illumi- 

 nated. The simplest observation which can be taken is made by placing 

 two sources of light (say, a candle and gas-lamp) opposite a white screen a 

 few feet off, and placing a stick in front of them, so that two shadows of 

 the stick may fall on the screen. The strongest light will cast the strongest 

 shadow ; and by moving this light away from the stick, keeping the sha- 



