1 6 S Bouguer and Mchille. [ Book 111. 



convincing proof of the velocity of light, and the mo- 

 tion of the earth in its orbit. Nor muft we forget M. 

 Bouguer's very curious and accurate experiments for 

 afcerta'ming the quantity of light which was loft by 

 reflexion, the moft decifive of which was by admitting 

 into a darkened chamber two rays of light, one of 

 which he contrived mould be reflected, and the other- 

 fall direct on the oppofite wall j then by comparing 

 the fize of the apertures, by which the light was ad- 

 mitted (that through which the direct ray proceeded 

 being much fmaller than that through which the, re- 

 flected ray was fuffered to pafs, and the illumination 

 on the wall being equal in both) he was enabled to 

 form an exact eftimate of the quantity of light which 

 was loft. To prove the fame effect with candles, he 

 placed himfelf in a room perfectly dark, with a book 

 in his hand, and having a candle lighted in the next 

 room, he had it brought nearer to him till he could juft 

 fee the letters, which were then twenty-four feet from 

 the candle. He then received the light of the candle 

 reflected by a looking-glafs upon the book, and he 

 found the whole diftance of the book from the fource 

 of the light (including the diftance from the book to 

 the looking-glafs) to be only fifteen feet; whence he 

 concluded, that the quantity of direct light is to that 

 of reflected as 576 to 225 ; and fimilar methods were 

 purfued by him for meafuring the proportions of light 

 in general *. 



The fpeculations of Mr. Melville, concerning the 

 blue fnadows which appear from opake bodies in the 

 morning and evening, when the atmofphere is ferene> 



* See an accurate defcription of M. Bouguer's infiruments, 

 Hijl. tf Optics, Per. vi. f. 7. 



are 



