174 Transactions of the American Institute. 



So valuable, indeed, is their guidance in nearly all precise and deli- 

 cate research in magnetism, that their attentive study has become 

 necessary to the further advancement of the science. Thinking, 

 therefore, that a method by which they can be firmly fixed on glass 

 plates, so that they can be securely kept for study and measurement, 

 serve for lantern slide and photographic negatives, would be welcome 

 to students of physics, I devised the following process. A clean 

 plate of thin glass is coated with a firm film of shellac, by 

 flowing over it a solution of this substance in alcohol, in the same 

 manner as a photographer coats a plate with collodion. After the 

 shellac film is hard, the plate is placed over the magnet, or magnets, 

 with its ends resting on slips of wood, so that the under surface of 

 the plate just touches the magnet. Iron filings, made from very soft 

 Norway iron, are now uniformly sifted over the plate by means of a 

 very fine sieve, and the magnetic curves are now developed by letting 

 fall, at different points on the plate, a light copper wire. The glass 

 is now lifted off the magnet and placed on the end of a cylinder of 

 pasteboard, which serves as a support in bringing it near the under 

 surface of a hot metal plate. Thus the shellac is uniformly heated, 

 and the iron filings, absorbing the radiation, sink into the surface and 

 are fixed. The plates can now serve (1) for the most accurate meas- 

 ures upon the magnetic field ; (2) for a photographic negative, which 

 in the printing-frame will produce the lines in white upon a dark 

 ground, giving most beautiful and distinct impressions ; or (3) they 

 can be used as " slides " for the lantern, and plates can thus be 

 secured for exhibition to the largest audiences; and this is important, 

 for it is not easy to obtain the best results in the quick experimenta- 

 tion required in a public lecture. 



My assistant will now place in the lantern a plate which has fixed 

 on it the curves of a straight bar magnet. Some of you will recog- 

 nize this as the original plate of the engraving which illustrates 

 Professor Tyndall's recent work, " Fragments of Science." From a 

 photographic print from this plate Dr. Tyndall made his engraving. 



Many philosophers have studied long and attentively these 

 graceful sweeping curves.* You may ask, what can one arrive at 

 by brooding over these forms \ Let us see. As you know, a mag- 

 netic needle carried along a curve will always point in its direction, 



♦Gilbert, Physiologia Nova deMagnete, 1600. Munchenbroek, Essais de Physiqne. Fnss, Coraent, 

 Petropolit. Lambert, ; Hist, de l'Acad. Roy. Sci., Berlin, 1776. Playfair. Robison's Mech. Phil., 

 vol. iv, p. 350. Leslie, Geometrical Analysis, Edin., 1821, p, 399. Dr. Roget, Jourl. Roy. Inst., vol. 

 i, p. 311, 1831, and in Nat. Phil, of Lib. Us. Know., vol. ii, article Magnetism, p. 19. Cellerier, in 

 De la Rives Traite d'Electriate, vol. i, p. 592. Faraday, Phil. Trans., 1852, p. 1. Pro. Royal Inst., 

 Jan.. 1852. 





