platinum and briefly examined, they are sent to Mr. Dollond, who then enters upon the discharge of his particular duties in the Committee, by cutting, examining, and even working them into telescopes. It is not, however, my place to detail this gentleman's exertions (as a member of the Glass Sub-committee) in the cause of science. They will, I trust, appear in due season; and I hope that the want of perfect success on my part will not long be a cause of delay.
§2. General qualities of the heavy Optical Glasses.
96. A great variety of glasses have been formed by the use of different proportions of ingredients. They vary importantly from each other, though by no means to the extent of the difference existing between any of them and flint glass. The specific gravity rises very high in borate of lead, consisting of single proportions, i. e. nearly 24 by weight of boracic acid and 112 of oxide of lead; it is often as high as 6.39 or 6.4, being double that of some specimens of flint glass. In silicated borate of lead, which, in addition to the former quantities, contains 16 parts, or a proportional of silica, it is about 5.44. As the proportion of oxide of lead diminishes, so also does the specific gravity lessen, and it is in some of the specimens as low as 4.2; still permitting by the proportions present such fusibility and other qualities as consist with the process described. The specific gravity of Guinaud's heavy flint glass is about 3.616; that of a specimen of ordinary flint glass 3.290; that of plate glass 2.5257; and that of crown glass 2.5448.
97. The refractive and dispersive powers of the glasses increase with their specific gravity, as was to be expected. The powers of two of them, namely, borate of lead and silicated borate of lead, consisting always, if not otherwise expressed, of single proportionals, have been ascertained by Mr. Herschel, and are as follows:—
| Bor.Lead. | Sil. Bor.Lead. | ||
| Angle of glass prism | 29° 6' | 30° 26' | |
| Refractive index for extreme red rays | μ= | 2.0430 | 1.8521 |
| Refractive index for maximum yellow | μ= | 2.0652 | 1.8735 |
| Refractive index for extreme violet | μ= | 2.1223 | 1.9135 |
| Dispersive index== | 0.0740 | 0.0703 | |
