42 
MR. FARADAY ON THE 
to effect it. As I have before said, we are in the course of our experiments 
only ; and up to the last have seen reason to vary the arrangements, and still 
intend to make alterations. Every thing agrees to convince me that the size 
of the plate is not a circumstance involving any additional difficulty; but that, 
on the contrary, it will probably be safer to make a large than a small experi- 
ment. We can at pleasure obtain a glass perfectly free from striae, unexcep- 
tionable in hardness, and with less colour than crown glass ; but it is the 
simultaneous absence of all striae and bubbles, with at the same time that 
degree of hardness and colour which will render the glass fit for optical pur- 
poses, that I am aiming at, and that I trust shortly to obtain. 
95. As soon as the plates of glass are removed from the platina 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 gen- 
tleman’s exertions (as a member of the Glass Subcommittee) 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 pro- 
portions 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 pro- 
cess described. The specific gravity of Guinand’s heavy flint glass is about 
3.616; that of a specimen of ordinary flint glass 3.290; that of plate glass 
2.525/ ; and that of crown glass 2.5448. 
9 7- The refractive and dispersive powers of the glasses increase with their 
