| Inches.# | ||||
| Light a direct | 10.70 | 1 | 1 | |
| Light a reflected by glass | 5 | 36.75 | 11.80 | 111.8 |
| 1 | 40.69 | 14.46 | 114.4 | |
| 4 | 43.46 | 16.50 | 116.5 | |
| 9 | 47.31 | 19.56 | 119.5 | |
| 6 | 50.31 | 22.12 | 122.1 | |
| 7 | 51.63 | 23.29 | 123.3 | |
| 3 | 52.69 | 24.26 | 124.2 | |
| 8 | 54.33 | 25.80 | 125.8 | |
| 2 | 54.56 | 26.02 | 126. |
The first column refers to the glasses below; the second gives the distance of the measuring flame b; the third, the preceding numbers squared and reduced to the direct light as unity; and the fourth, consequently, the proportion of the light a reflected by the first surface of each glass. No. 5 was glass consisting of 1 proportional of oxide of lead, ½ a proportional of silica, and 1½ proportional boracic acid. No. 1 was composed of 1 oxide of lead, 1 silica, and 1½ boracic acid. No. 4, of 1 oxide of lead, 1½ silica, and 1½ boracic acid. No. 9 was flint glass; No. 6, 7 and 3, different pieces of crown glass; and No. 8 and 2, different pieces of plate glass. 1, 3, 5, 6 and 7, were natural surfaces; 2, 4, 8 and 9, polished surfaces.
The deficiency of light resulting from the increased reflecting power, though considerable, may easily be compensated for by slightly increasing the area of the plate; and the power of obtaining plates of any size is professed to be given by the general process: but whether that expedient involves any other objections, it will be for the optician to determine.
101. In hardness, these glasses differ from each other as much as in any other quality, and indeed more. The borate of lead is very soft; the biborate of lead is harder, and the triborate equal to flint glass in hardness. The silicated borate of lead is softer than flint glass; but the glass consisting of proportional oxide of lead, 1 of silica, and 1½ proportional of boracic acid, is as hard as ordinary flint glass, at the same
