Optics. 369 



Tscribed particularly the zenith sector, the quadrant, the repeating circle, 

 the great declination circles of Troughton, and the circle of Reichenbach's 

 construction ; and instituted a comparison between the two last as the de- 

 cHnation instruments which at present are principally used in European 

 observatories. 



Art. XXVI.— scientific INTELLIGENCE. 



I. NATURAL PHILOSOPHY. 



OPTICS 



1. On the Manufacture of Glass for optical purposes. By Mr Fara- 

 day, Esq. F. R. S. — The following is an abstract of Mr Faraday's paper 

 on this important subject. The author, being intrusted with the su- 

 perintendence of the experimental part of the manufacture of the glass, 

 conceives it to be his especial duty, at the present stage of the inquiry, 

 to give an account of what has been done in his department; for 

 although the investigation is yet far from being completed, he trusts 

 that a decided step has now been made in the manufacture of glass 

 for optical purposes, and that it is due to the Society, as well as to 

 the government, to render an account of the results hitherto obtained. 

 The author begins this account by a statement of the usual defects inci- 

 dent to glass, which destroy the regularity of its action on light. These 

 are, on the one hand, streaks, striai, veins, and tails ; and, on the other 

 hand, minute bubbles ; the> former arising from the want of homo- 

 geneity — the latter from the intermixture of air. Of these, the first class 

 of defects constitute the most serious evil, as they interfere with the recti- 

 lineal course of the rays of light while traversing the glass ; while the lat- 

 ter are injurious merely from the interception of the rays, and their dis- 

 persion in all directions. The greater the difference in specific gravity of 

 the ingredients of the glass, the greater is the tendency to form stria; when 

 they are fused together : hence flint glass, which contains a large propor- 

 tion of lead, is more liable to this defect than either crown or plate glass. 

 After numerous trials of materials different from those which enter into 

 the composition of the ordinary kinds of glass, borate of lead and silica 

 were fixed upon as the most eligible ; and as near an approximation as 

 possible to a definite chemical union of their elements was arrived at, by 

 taking single proportionals of each, and endeavouring to procure them, 

 previous to combination, in the greatest possible state of purity. The 

 oxide of lead was obtained from the nitrate of the metal previously crystal- 

 lized. The boracic acid was also selected from the purest crystals afford- 

 ed by the manufacturer, and carefully tested to ascertain its freedom from 

 foreign matters. The silica employed was that of flint-glass-makers' sand, 

 well washed and calcined, and freed from iron by nitric acid. It was af- 

 terwards combined with protoxide of lead. These materials were then 

 mixed, in the proportion of 154.14> parts ot nitrate of lead, 24- of siHcate of 

 lead, and 42 of crystallized boracic acid, and melted together in a separate 

 furnace, adapted expressly for this preliminary operation, and of which a 

 minute description is given. A tray was then prepared of thin laminae of 

 platina — all the apertures of which were carefully closed by soldering — 



