VOL. LXXXIV.] PHILOSOPHICAL TKANSACTIONS. 36Q 



was found that the loss of light in its passage through this pane of glass, taking 

 a mean of all the experiments, was .1973 parts of the whole quantity that im- 

 pinged against it ; the variations in the results of the various experiments being 

 from .1720 to .2108. In 4 experiments, with another pane of the same kind of 

 glass, the loss of light was .1836 ; .1732; .2056; and .1853 ; the mean .1869. 



When the two panes of this glass were placed before the lamp b, at the same 

 time, but without touching each other, and the light was made to pass through 

 them both, the loss of light, in 4 different experiments, was .3O89 ; .3259 ; 

 .3209 ; and .3180; the mean .3184. — With another pane of glass of the same 

 kind, but a little thinner, the mean loss of light, in 4 experiments, was .1813. 

 — With very a thin clean, pane of clear, white, or colourless window-glass, not 

 ground, the loss of light, in 4 experiments, was .1324; .1218; .1213; and 

 .1297 ; the mean .1263. When the experiment was made with this same pane 

 of glass, a very little dirty, the loss of light was more than doubled. — Might 

 not this apparatus be very usefully employed by the optician, to determine the 

 degree of transparency of the glass he employs, and direct his choice in the pro- 

 vision of that important article in his trade ? 



In making these experiments, a great deal of the trouble may well be spared, 

 for there is no use whatever in bringing the two lamps a and b to burn with the 

 same degree of brilliancy ; all that is necessary being to bring the shadows to be 

 of the same density, with the glass, and without it, noting the distance of the 

 lamp B in each case, the lamp a remaining immoveable in its place ; for the re- 

 lative quantity of light lost will ever be accurately shown by the ratio of the 

 squares of those distances, whatever be the relative brilliancy with which the 

 2 lamps burn. The experiment is more striking, and the consequences drawn 

 from it rather more obvious, when the lamps are made to burn with equal flames; 

 otherwise that equality is of no real advantage. 



The 3d head of experiments is titled, " On the Loss of Light in its Re/lection 

 from the Surface of a Plane Glass Mirror" 



In these experiments the method of proceeding was much the same as in 

 those just mentioned. The lamps A and b burning with clear, bright, and 

 steady flames, were placed before the field of the photometer, and one of them 

 was moved backwards and forwards till the illuminations of the shadows in the 

 field of the instrument were found to be precisely equal. The distance of the 

 lamp B being then noted, this lamp was removed, and a mirror being put in its 

 place, but nearer the field of the photometer, the lamp was so placed that its 

 rays, striking the centre of the mirror, were reflected against the field of the 

 photometer, where, by bringing the lamp nearer to, or removing it farther from 

 the mirror, the illumination of the field by those reflected rays was now brought 

 to be in equilibrium with the illumination of the standard lamp, and then the 



VOL. XVII, 3 B 



