370 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



distance of the lamp from the centre of the mirror, and the distance from thence 

 to the centre of the field, were carefully measured and noted. These 2 distances 

 added together, gave 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 re- 

 flected 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 illumi- 

 nated by the direct rays of the same lamp, the lamp must be brought nearer to 

 the field. It is also 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 illuminate equally the 

 field of the photometer, will be an accurate measure of the loss of the light in 

 reflection. 



The mean of 5 experiments, made with an excellent mirror, gave for the loss 

 of light .3494 ; and hence it appears, that more than ^ part of the light, which 

 falls on the best glass mirror that can be constructed, is lost in reflection. The 

 loss with mirrors of indifferent quality, is still more considerable. With a very 

 bad common looking-glass the loss, in one experiment, appeared to be .48 16 

 parts ; and with another looking-glass it was .4548 parts in one experiment, and 

 .4430 in another. He would have made an experiment to determine the loss of 

 light in its reflection from the surface of a plane metallic mirror, but he had no 

 such mirror at hand. The difi^erence of the angles of incidence at the surface of 

 the mirror, within the limits employed, namely, from 45° to 85°, did not appear 

 to aflfect, in any sensible degree, the results of the experiments. He also found 

 on trial, that the effect produced by the difference of the angles at which light 

 impinges against a sheet of transparent glass through which it passes, is, within 

 the limits of 40° or 50° from the perpendicular, but very trifling. 

 The 4th head of experiments is titled, " On the Relative Quantities of Oil con- 

 sumed, and of Light emitted, by an Argand's Lamp, and by a Lamp on the 

 Common Construction, with a Ribband PVick." 



The brilliancy of the Argand's lamp is not only unrivalled, but the invention 

 is in the highest degree ingenious, and the instrument useful for many purposes ; 

 but still, to judge of its real merits as an illuminator, it was necessary to know 

 whether it gives more light than another lamp in proportion to the oil consumed. 

 This point was determined in the following manner. Having placed an Argand's 

 lamp, well trimmed, and burning with its greatest brilliancy, before the photo- 

 meter, and over against it a very excellent common lamp, with a ribband wick, 

 about an inch wide, and which burned with a clear bright flame, without the 

 least appearance of smoke, the intensities of the light emitted by the 1 lamps 

 were to each other as 17956 to 9063 ; the densities of the shadows being equal 



