of the Light emitted by Luminous Bodies. 31 



mirror, and the distance from thence to the centre of 

 the field, were carefully measured and noted. These 

 two distances added together was the real distance 

 through which the rays passed in order to arrive at 

 the field of the photometer. 



Now, as there is always a loss of light in reflection, 

 it is evident that the reflected rays must come to the 

 field of the photometer weakened, and that in order to 

 illuminate this field by these reflected rays as strongly 

 as it was illuminated by the direct rays of the same 

 lamp, the lamp must be brought nearer to the field. 

 It is likewise evident, from what has already been said, 

 that the ratio of the squares of those distances of the 

 lamp when its rays pass on directly, and when they 

 arrive after having been reflected are found to illu- 

 minate equally the field of the photometer, will be 

 an accurate measure of the loss of the light in re- 

 flection. 



The following table will show the results of five 

 experiments with a small but most excellent glass 

 mirror made by Ramsden. This mirror, which makes 

 part of an optical instrument I caused to be con- 

 structed in London about twelve years ago, is 7 inches 

 long and 5! inches wide, and I suppose is as perfect as 

 ever glass mirror was of that size. 



To facilitate the comparison of the results of the 

 experiments, the lamp B at the beginning of each 

 experiment (when the intensity of its direct rays was 

 compared with the intensity of the standard lamp) was 

 placed at the distance of 100 inches, the standard lamp 

 being occasionally moved, in order to produce an 

 equality of the shadows. 



