According to the experiments of M. Bouguer, the thickness of a line of glass destroys 2/7 of light, and consequently the diminution would be made in the following proportion: Thickness, 1, 2, 3, 4, 5, 6 lines Diminution, 2/7, 10/49, 50/343, 250/2401, 1250/16807, 6250/117649 So that by the sum of these six terms we should find, that the light which passes through six lines of glass would lose 102024/117649, that is about 10/11 of its quantity. But it must be considered, that M. Bouguer makes use of glasses which are but little transparent, since he has observed, that the thickness of a line of these glasses destroys 2/7 of the light. By the experiments which I have made on different kinds of white glass, it has appeared to me that the light diminishes much less. These experiments are easy to be made, and are what all the world may repeat. In a dark chamber, whose walls were blackened, and which I made use of for optical experiments, I had a candle lighted of five to the pound; the room was very large and the candle the only light in it; I then tried at what distance I could read by this light, and found that I read very easily at 24 feet four inches from the candle. Afterwards, having placed a piece of glass, about a line thick, before it, at two inches distance, I found that I still read very plainly at 22 feet nine inches; and substituting to this glass another piece of two lines in thickness and of the same glass, I read at 21 feet distance from the candle. Two of the same glasses joined one to the other, and placed before the candle diminished the light so much that I could only read at 17-1/2 feet distance; and at length, with three glasses, I could only read at 15 feet. Now the light of a candle diminishing as the square of the distance augments, its diminution should have been in the following progression, if glasses had not been interposed: 2--24-1/3. 2--22-3/4. 2--21. 2--17-1/2. 2--15, or 592-1/9. 517-9/15. 441. 306-1/4. 225. Therefore the loss of the light, by the interposition of the glasses, is in the following progression: 84-79/144. 151. 285-7/9. 367-1/4. From hence it may be concluded, that the thickness of a line of this glass diminishes only 84/592 of light, or about 1/7; that two lines diminishes 157/592, not quite 1/4 and three glasses of two lines 397/592, i. e. less than 2/3. As this result is very different from that of M. Bouguer, and as I was cautious of suspecting the truth of his experiments, I repeated mine with common glass. For long telescopes water alone can be used; and it is still to be feared that an inconveniency will subsist, from the opacity resulting from the quantity of liquor which fills the interval between the two glasses.