3(38 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



some lolerab!}' accurate measure. An accurate determination of the relative in- 

 tensity of" the sun's or moon's light, when seen at different heights above the 

 horizon, or when seen from the top, and from the bottom of a very high moun- 

 tain, in very clear weather, would probably lead to a discovery of the real amount 

 of the resistance of the air to light. 



The next head is intitled, •' On the Loss of Light in its Passage throu°h Plates 

 or Panes of different Kinds of Glass." 



In these experiments Count R. proceeded in the following manner. Having 

 provided 2 equal Argand's lamps, a and b, well trimmed, and burning with very 

 clear bright flames, they were placed over against each other before the photo- 

 meter, each at the distance of 100 inches from the field of the instrument, and 

 the light of B was brought to be of the same intensity as that of a, or the sha- 

 dows were brought to be of the same density, which was done by lengthening 

 or shortening the wick of the lamp b, as the occasion required. This done, and 

 the 2 lamps now burning with precisely the same degree of brilliancy, a pane of 

 fine, clear, transparent,, well-polished glass, such as is commonly used in the con- 

 struction of looking-glasses, 6 inches square, placed vertically on a stand, in a 

 small frame, was interposed before the lamp b, at the distance of about 4 feet 

 from it, and in such a position that the light emitted by it was obliged to go 

 perpendicularly through the middle of the pane, in order to arrive at the field 

 of the photometer. The consequence of this was, that the light of the lamp 

 B being diminished and weakened in its passage through the glass, the illumi- 

 nations of the shadows in the field of the photometer were no longer equal, the 

 shadow corresponding to the lamp a being now less enlightened by the light of 

 the lamp b, than the shadow corresponding to the lamp b was enlighted by the 

 undiminished light of the lamp A. 



To determine precisely the exact amount of this diminution of the light of 

 the lamp b, which was the main object of the experiment, nothing more was 

 necessary than to bring this lamp nearer to the field of the photometer, till its 

 light passing through the glass should be in equilibrium with the direct light of 

 the lam[) a ; or, in other words, till the equality of the shadows should be re- 

 stored ; and this he found actually happened when the lamp b, from 100 inches, 

 was brought to the distance of 90. 2 inches from the field of the photometer. 

 Now as it has already been shown that the intensities of the lights are as the 

 squares of their distances from the field of the photometer, the illuminations 

 being equal at that field, it is evident that the light of the lamp B was diminished, 

 in this experiment, in its passage through the pane of glass, in the ratio of 100^ 

 to 90.2'\ or as l to .8136 ; so that no more than .813(5 parts of the light which 

 impinged against the glass found its way through it ; the other .1804 parts being 

 dispersed and lost. This experiment was repeated no less than 10 times; and it 



