6 Experiments on the Relative Intensities 



always be found exactly in the line of reflection of the 

 other; otherwise the rays from the different lights 

 falling upon the paper, and consequently upon the 

 shadows, at different angles, will render the experi- 

 ment fallacious. 



When the intensity of one strong light is compared 

 with the intensities of several smaller lights taken 

 together, the smaller lights should be placed in a line 

 perpendicular to a line drawn to the centre of the 

 paper, and as near to each other as possible ; and it is 

 likewise necessary to place them at a greater distance 

 from the paper than when only single lights are com- 

 pared. 



In all cases, it is absolutely necessary to take the 

 greatest care that the lights compared be properly 

 trimmed, and that they burn clear and equally, other- 

 wise the results of the experiments will be extremely 

 irregular and inconclusive. It is astonishing what a 

 difference there is in the quantities of light emitted 

 by the same candle, when it burns with its greatest 

 brilliancy, and when it has grown dim for want of 

 snuffing. But as this diminution of light is progres- 

 sive, and as the eye insensibly conforms to the quan- 

 tity of light actually present, it is not always taken 

 notice of by the spectators. It is nevertheless very 

 considerable in fact, as will be apparent to any one 

 who will take the trouble to make the experiment; 

 and so great is the fluctuation in the quantity of light 

 emitted by burning bodies, lamps or candles, in all 

 cases, even under the most favourable circumstances, 

 that this is the source of the greatest difficulties I have 

 met with in determining the relative intensities of 

 lights by the method here proposed. 



