of the Light emitted by Liiminous Bodies. 27 



clear weather, would probably lead to a discovery of 

 the real amount of resistance of the air to light.* 



Of the Loss of Light in its Passage through Plates or 

 Panes of different Kinds of Glass. 



In these experiments I proceeded in the following 

 manner. Having provided two 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 photometer, 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 shadows 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 two lamps now burn- 

 ing with precisely the same degree of brilliancy, a 

 pane of fine, clear, transparent, well-polished glass, 

 such as is commonly made use of in the construction 

 of looking-glasses, six inches square, placed vertically 

 upon a stand, in a small frame, was interposed before 

 the lamp B at the distance of about four 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 pho- 



* This method of ascertaining the diminution of light in passing through the 

 atmosphere was proposed, and put in practice, many years ago, by an ingenious 

 French philosopher, M. Bouguer, of the Royal Academy of Sciences. See 

 Traite d'Optique pour la Gradation de la Lumiere : Ouvrage posthume de M. Bou- 

 guer, de 1' Academic Royale des Sciences, etc. Published at Paris by the Abbe 

 de la Caille, in the year 1760. 



When this paper was written, I had not seen that most ingenious and learned 

 dissertation. It did not come into my hands till a few months ago (in Novem- 

 ber, 1801) when, being at Paris, my worthy and respectable friend the Senator 

 Laplace procured it for me. 



