As this is a circumstance connected with the construction of reflecting telescopes, it may not be improper, in this place, to state some of the results of the accurate experiments of M. Bonguer on this subject. This philosopher ascertained that of the light reflected from mercury, or quicksilver, more than _one-fourth_ is lost, though it is probable that no substances reflect more light than this. The rays were received at an angle of eleven and a half degrees of incidence, measured from the surface of the reflecting body, and not from the perpendicular. The reflection from _water_ was found to be almost as great as that of quicksilver; so that in very small angles it reflects nearly three-fourths of the direct light. This is the reason why so strong a reflection appears on water, when one walks, in still weather, on the brink of a lake opposite to the sun. The direct light of the sun diminishes gradually as it approaches the horizon, while the reflected light at the same time grows stronger; so that there is a certain elevation of the sun in which the united force of the direct and reflected light will be the greatest possible, and this is when he is twelve or thirteen degrees in altitude. On the other hand, light reflected from water at _great angles_ of incidence is extremely small. When the light was perpendicular, it reflected no more than the thirty-seventh part which mercury does in the same circumstances, and only the fifty-fifth part of what fell upon it in this case. Using a smooth piece of glass, one line in thickness, he found that, when it was placed at an angle of fifteen degrees with the incident rays, it reflected 628 parts of 1000 which fell upon it; at the same time, a metallic mirror which he tried in the same circumstances, reflected only 561 of them. At a less angle of incidence much more light was reflected; so that at an angle of three degrees, the glass reflected 700 parts, and the metal something less, as in the former case. The most striking observations made by this experimenter relate to the very great difference in the quantity of light reflected at different angles of incidence. He found that for 1000 incident rays, the reflected rays, at different angles of incidence, were as follows. Angles of Rays reflected Rays reflected incidence by water by glass 5° 501 549 10 333 412 15 211 299 30 65 112 50 22 34 70 18 25 90 18 25 With regard to such mirrors as the specula of reflecting telescopes, it will be found, in general, that they reflect little more than the _one half_ of the rays which fall upon them.