Mathematical Proceedings Volume 4 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1883 Excerpt: ...when we use rockcrystal instead of crown-glass. Most likely, as I have said before, at that date (1821) the aim of inventors was to make flint-glass of much higher specific gravity, and so of much higher dispersive power. In the Bakerian Lecture for 1829 Faraday describes the experiments by which he, in conjunction with a committee of the Royal Society, had made a flint-glass of specific gravity 5 3, employing oxide of lead; and it must be in reference to this that it is said in the Astron. Nachr., vol. VII., in the year 1829, that he had made an object-glass of 2J inches diameter with a flint-glass of that specific gravity, which had succeeded very well; but I am sorry to say that want of knowledge prevents me giving any farther information as to what was done in that direction. Still it would not be difficult to calculate the radii of the surfaces on Herschel's rules; and I apprehend that this is what my optician has done. It will only be necessary to extend Herschel's table, which now extends from-50 up to '75 dispersive ratio down to-40, at most. I am inclined therefore to think that M. Cauchoix's idea that he could shorten the focal length of object-glasses by using quartz, was not founded on theory, but only on some such practical experience as leads opticians to shorten telescopes when used for terrestrial objects; and I think that the only real advantage to be gained by the use of this material is from its transparency. I am not sure how far this advantage in the present state of glass manufacture will be actually realized. Rock-crystal is undoubtedly a most beautifully transparent substance, but so is the best modern glass; and when employing alternately the old and actually greenish, though I must say well-made, object-glass, and the new one compo...