﻿Photographing, and exhibiting the Magnetic Spectra. 477 



" the lines of magnetic force," given to these spectra, even when 

 merely regarded as conventional symbols, an importance which 

 has been fully shown (especially) by Faraday, who was guided 

 by their consideration to some of his most important discoveries. 

 They have thus risen to so high a theoretical importance that a 

 method which will fix them without danger of distortion, photo- 

 graphically reproduce them and readily serve to exhibit them to 

 the largest audiences, will, I imagine, be acceptable to both in- 

 vestigators and lecturers. 



The only process of fixing these spectra known to me is that 

 practiced by De Haldat and Faraday, which, however, is but an 

 application to the magnetic spectra of the method previously in- 

 vented by Savart for preserving the Chladni figures of vibrating 

 plates. In this process the spectra, produced in the usual man- 

 ner either on glass or cardboard, have pressed upon them a sheet 

 of paper coated with mucilage, to which the filings adhere. In 

 this operation of the transfer many particles are deranged from 

 their positions, and the figures are yet more distorted by the 

 shrinkage of the wet paper, and are therefore not fit to serve in 

 measures of precision; while the impressions cannot be exhibited 

 with much more facility than the originals. 



My process is as follows : — 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 photographic 

 plate is coated with collodion. After the plate has remained a 

 day or two in a dry atmosphere, it 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. Fine iron-filings, 

 produced by "draw-filing" Norway iron which has been repeat- 

 edly annealed, are now sifted uniformly over the film of lac by 

 means of a fine sieve. The spectrum is then produced on vibra- 

 ting the plate by letting fall vertically upon it at different points 

 a light piece of copper wire. The plate is now cautiously lifted 

 vertically off the magnet and placed on the end of a cylinder of 

 pasteboard, which serves as a support in bringing it quite close 

 to the under surface of a cast-iron plate (1 foot diameter, J inch 

 thick) which has been heated over a large Bunsen-fiame. Thus 

 the shellac is uniformly heated, and the iron-filings, absorbing 

 the radiation, sink into the softened film and are " fixed." 



I generally allow the heat to act until the metallic lustre of 

 the filings has disappeared by sinking into the shellac, and the 

 film appears quite transparent. This degree of action is neces- 

 sary when photographic prints are to be made from the plate; 

 but when they are to be used as lantern-slides I do not carry the 



* The shellac dissolved in strong alcohol is allowed to stand a week or 

 more, and the clear supernatant solution is then decanted. 



