Luminous Intensity of Light. 



361 



obtaining uniform results with the Act-of-Parliament candle. A true 

 sperm-candle is made from a mixture of refined sperm with a small pro- 

 portion of wax, to give it a certain toughness, the pure sperm itself being 

 extremely brittle. The wick is of the best cotton, made up into three 

 cords and plaited. The number of strands in each of the three cords 

 composing the wick of a six-to-the-pound candle is seventeen, although Mr. 

 Sugg says there does not appear to be any fixed rule, some candles having 

 more and others less, according to the quality of the sperm. Sperm-candles 

 are made to burn at the rate of one inch per hour, and the cup should be 

 clean, smooth, and dry. The wick should be curved slightly at the top, 

 the red tip just showing through the flame, and consuming away without 

 requiring snuffing. To obtain these results, the tightness of the plaiting 

 and size of the wick require careful attention ; and as the quality of the 

 sperm differs in richness or hardness, so must the plaiting and number of 

 strands. A variety of modifying circumstances thus tend to affect the 

 illuminating power of a standard sperm-candle. These difficulties, how- 

 ever, are small compared with those which have resulted from the sub- 

 stitution of paraffin &c. for part of the sperm ; and Mr. Sugg points out 

 that candles can be made with such combinations of stearin, wax, or sperm, 

 and paraffin, as to possess all the characteristics of sperm-candles and yet 

 be superior to them in illuminating-power; while, on the other hand, 

 candles made from the same materials otherwise combined are inferior. 

 When, in addition to this, it is found that candles containing paraffin re- 

 quire wicks more tightly plaited and with fewer strands than those suitable 

 for the true sperm-candle, our readers will be enabled to judge of the 

 almost insurmountable difficulties which beset the present system of 

 photometry. 



But assuming that the true parliamentary sperm-candle is obtained, 

 made from the proper materials, and burning at the specified rate, its 

 illuminating-power will be found to vary with the temperature of the 

 place where it has been kept, the time which has elapsed since it was 

 made, and the temperature of the room wherein the experiment is tried. 



The Rev. W. R. Rowditch, in his work on ' The Analysis, Purification 

 &c. of Coal-gas,' enters at some length into the question of test-candles, 

 and emphatically condemns them as light-measurers. One experiment 

 quoted by this author showed that the same gas was reported to be 14*63 

 or 17*36 candle-gas, according to the way the experiment was conducted. 



The present writer 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 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 description of the sperm -candle now employed, to 

 make a standard which would bring his photometric results into relation 



