of the Light emitted by Luminous Bodies. 29 



was again restored, when the lamp B arrived at its 

 former station, 100 inches from the field of the pho- 

 tometer. 



This experiment I repeated no less than 10 times, 

 and found the loss of light in its passage through this 

 pane of glass, taking a mean of all the experiments, to 

 be .1973 parts of the whole quantity that impinged 

 against it ; the variations in the results of the various 

 experiments being from .1720 to .2108. 



In four experiments, with another pane of the same 

 kind of glass, the loss of light was .1836, .1732, .2056, 

 and .185 3; 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 made to pass through them 

 both, the loss of light in four different experiments 

 was .3089, .3259, .3209, and .3180; mean, .3184. 



With another pane of glass of the same kind, but 

 a little thinner, the mean loss of light in four experi- 

 ments was .1813. 



With a very thin, clean pane of clear white or 

 colourless window-glass, not ground, the loss of light 

 in four experiments was .1324, .1218, .1213, and .1297 ; 

 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 transpar- 

 ency of the glass he employs, and direct his choice in 

 the provision 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 what- 

 ever in bringing the two lamps A and B to burn with 



