1 87 1.] War Science. 53 



The instrument, as our readers will suppose, is fitted in a 

 darkened chamber, and, moreover, screened with black 

 curtains, to shelter it from any stray beams of light ; at one 

 end of a horizontal bar, some three or four feet in length, 

 is planted the standard light with which the candles are to be 

 compared, and which may itself consist of a candle of known 

 luminosity or of an oil-lamp of uniform light-giving qualities. 

 At the other end of the beam are one or more candlesticks, 

 for the reception of the specimens, fitted upon a stand, which 

 forms, in fact, one side or scale of a balance, the functions 

 of which shall presently be explained. The horizontal beam 

 is marked off into inches or degrees, and adls in the capacity 

 of a tramway, upon which a little waggon bearing the 

 photometer runs freely to and fro when moved by the hand; 

 the essential part of this instrument is a paper disc, of which 

 the centre has been stained with oil so as to appear trans- 

 parent, whenever there happens to be a greater amount of 

 light behind it than before it. This paper disc mounted 

 upon the waggon, separates, as it were, the two flames from 

 each other (that of the standard light and that under 

 examination), and is moved to and fro along the beam, 

 sometimes towards one flame and sometimes towards 

 the other, until a spot is found where the paper diaphragm 

 appears opaque all over and the transparency of the centre 

 is invisible, showing, therefore, that the amount of illumina- 

 tion proceeding from each source is at that point identical. 

 This result obtained, the degree marked upon the beam is 

 carefully noted; if the photometer is exactly midway between 

 the two lights, when equal illumination is shown, then we 

 know that the standard and experimental flames are 

 identically alike in intensity ; but if, on the contrary, the 

 diaphragm happens to' be farther from the standard than the 

 other, then we know that the light under examination is the 

 weaker of the two, for it has been necessary to approach 

 nearer to it to obtain the same amount of illumination as 

 that afforded by the standard at a longer distance off. The 

 degrees marked upon the bar will give the precise result of 

 the investigation, showing at once the comparative power of 

 the experimental flame, and in what respect or degree it is 

 greater or less than the standard ; but the photometric test 

 does not end here, for it not unfrequently happens that 

 superior illumination is simply due to an extravagant rate of 

 burning of the material of which the candle is composed ; 

 and a second question thus arises, as to how much of the 

 candle has been consumed during the period of the experi- 

 ment. This is ascertained by the balance arrangement above 



