16 REPORT — 1842. 



three or four feet wide, the whole breadth of the window, by the mere inclination of 

 the prism, which had the effect of producing a narrowing, facing the light. He had 

 obtained 200 or 300 results, which He had not had any leisure to group ; but he would 

 mention some of the general results. When nitrate of lead was thrown into combustion, 

 remarkably tine lines were produced in the spectrum. The luminous line, D, of Frau-n- 

 hofer, existed in almost every substance, especially in all into which soda entered, par- 

 ticularly in the flame of a common tallow candle; probably owing to the muriate of 

 soda existing in the tallow. The hydrate of strontites gave the lines very remarkably in 

 yellow and green. The iodide of mercury did the same. Also in that remarkable 

 substance, the lithoxanthemate of ammonia, first discovered and published by Mr. Fox 

 Talbot, the fine lines were seen throughout the whole length of the spectrum; and 

 there was a remarkable blue band, which he (Sir David Brewster) bad not distinctly 

 recognized in any other flame. Indigo gave fine green and orange lines at equal di- 

 stances from the D of Fraunhofer. Prussian blue did the same; calomel, nitrate of 

 magnesia, litharge, also showed lines; the sulpho-cyanite of potash gave a violet and 

 orange flame, with the lines extremely distinct. He hoped, at the next year's meeting 

 of the Association, to be able to embody these various results in a regular report. 



On the Improvement of the Telescope. By H. Fox Talbot, F.R.S. 



Mr. Fox Talbot said, that this subject occurred to him about two years ago, when 

 the Earl of Rosse (then Lord Oxmantown) was making much larger specula for re- 

 flecting telescopes than had ever been obtained before ; and he thought, if once we 

 had a very large and perfect speculum, it might be possible to multiply copies of it by 

 galvanic means. He had observed, that if an electrotype cast were taken from a per- 

 fectly polished surface, the cast was also perfectly polished; so that no defect of form 

 from this cause could have an injurious effect on the speculum. The great and ob- 

 vious defect was, that electrotypes were in copper, which reflected but little light. 

 He mentioned these ideas to Prof. Wheatstone, to whom the same views had occurred 

 previously, as he showed Mr. Talbot a paper which he had drawn up some few 

 months before, in which he suggested the making specula of platina, palladium, silver 

 or nickel, by precipitating a sufficient thickness of these metals upon the mould to ob- 

 tain a perfect surface, and afterwards precipitating a less valuable metal, as copper, to 

 form the body of the new speculum. 



Though it had occurred to Mr. Talbot to precipitate white metals, yet he did not 

 think that platina would have a sufficiently beautiful white metallic polish. Prof. 

 Wheatstone had, however, made choice of platina; and, varying the quantity till he 

 found the required proportion, he obtained a mirror in platina, which appeared to him 

 (Mr. Talbot) to have quite brilliant polish enough, and to be white enough to answer 

 the purpose ; and he considered, therefore, that Prof. Wheatstone had proved, that, at 

 least in one form, the specula of telescopes might be made by voltaic precipitation. 

 His own idea was, that it might be possible to whiten the surface of the copper without 

 injuring the form ; and, therefore, having obtained a speculum in very bright, polished 

 copper, be (Mr. Talbot) whitened it, and transformed it into sulphurct of copper, by 

 exposing it for a minute to the vapour of hydrosulphuret of ammonia, which did not 

 injure its polish, but after passing through a series of varied colours (scarlet, blue, 

 &c.) rendered it very white; and after having retained this speculum about a year, he 

 did not perceive the smallest alteration in any respect. This, therefore, appeared to him 

 a mode bv which important results could be obtained. There was no danger of such 

 a speculum being oxidated by the air, since it was already in combination with sul- 

 phur, a more powerful chemical affinity than oxygen. 



For the last year, perhaps, nothing further had been done, either by Prof. Wheat- 

 stone or himself; but lately, being at Munich, he visited Prof. Steinheil, and saw his 

 inventions, and learned from him that he had discovered a method of making specula 

 by the electrotype. It so happened, that both Prof. Steinheil and himself had pub- 

 lished their respective methods about a month or six weeks before; the Professor 

 having read a communication on the subject before the Academy of Sciences at Mu- 

 nich, and printed it, and Mr. Talbot having published his in England. Their modes 

 were, however, different, as Prof. Steinheil precipitated gold upon the speculum which 

 was to be copied, and having precipitated a certain thickness of gold, he then precipi- 

 tated copper on the back of the gold, to give it sufficient thickness. He (Mr. Talbot) 



