﻿478 Dr. A. M. Mayer on a Method of Fixing, 



heating so far. After the plate has cooled, it is allowed to fall 

 upon its ends on a table, so that any filings which have not ad- 

 hered may be removed. 



A short experience will give the proper strength of shellac 

 solution to obtain a film so thick as just to be sufficient to hold 

 the filings, and the requisite amount of heat to firmly cement 

 them, without injuring the transparency of the film. 



The plates can now serve (1) for the most accurate measures 

 upon the magnetic field, (2) for a photographic positive, which 

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

 dark ground, giving most be.autiful and distinct impressions*; 

 or (3) if it is required to exhibit these figures to an audience, the 

 plates are provided with glass covers, kept from touching the 

 spectra by intervening slips of cardboard, and there result 

 " slides " in every way fit for giving a fine exhibition when the 

 images are projected upon a screen. I have thus obtained 

 images clear and sharp of over 12 feet diameter. 



By this process many plates have been producedf, showing 

 the action of single magnets of various forms, and of juxtaposed 

 bars, as well as the effects of electric currents led by wires through 

 holes drilled in the plates. Those exhibiting the inductive action 

 of magnets on bars of soft iron and the interaction of magnets 

 and electric currents are peculiarly interesting. An approximate 

 representation of the resultant lines of the terrestrial magnetic 

 action has been obtained by magnetizing equably tempered steel 

 disks of from 2 to 3 inches, and even more, in diameter. The 

 magnetic axis or axes of these disks are predetermined by making 

 them the continuations of the axes of very powerful electro-mag- 

 nets, terminated with cones of soft iron with slightly rounded 

 apices. The arcs of the great circles including the terrestrial 

 magnetic poles having been calculated, the axes of the electro- 

 magnets are inclined to that angle, while the steel disk is held 

 close to their poles. On passing the current the disk is magne- 

 tized, and we have an approximate representation of a section of 

 the earth's magnetic effect. These results, when viewed as pho- 

 tographic prints or as exhibited by the lantern, are so beautiful 

 and instructive as to appear to me to warrant this somewhat 

 formal description of the process of their production. 

 December 18/0. 



* Photographic prints from a series of eight of these plates I have pre- 

 sented to : — Harvard College ; American Academy « of Sciences ; Sheffield 

 Scientific School ; Columbia College ; Stevens Institute of Technology, 

 Hoboken; Lehigh University, Pa.; American Philosophical Society; 

 Franklin Institute ; Peabody Institute, Bait. ; Smithsonian Institution ; 

 Chicago Academy of Sciences; and to the University of Virginia, — where 

 they can be examined by the readers of this paper. 



t Several of these are 16 inches long by 10 wide. 



