TRANSACTIONS OF THE SECTIONS. 17 



should have thought beforehand that gold would not reflectlight enough to be available; 

 but Prof. Steinheil informed him he had found, by careful experiment, that it reflected 

 more light than polished steel. He allowed Mr. Talbot to look through a Gregorian 

 reflecting telescope, of which the speculum was a common one, but gilded, and he 

 found the image perfectly clear and well-defined, though a slight tinge of yellow was 

 thrown over all the objects. Prof. Steinheil said, that in the course of a year he should 

 have a very large telescope, furnished not only with a speculum, but also with other 

 apparatus, voltaically formed, so that telescopes might be made all from a good model, 

 so as to insure greater accuracy of proportions ; and in this way even very large tele- 

 scopes might be constructed- at a comparatively trifling expense. With reference to 

 precipitating copper on the back of the gold, the Professor had a simple expedient for 

 securing adhesion. He first precipitated gold from the cyanide of gold, and he mixed 

 with it cyanide of copper, and kept gradually increasing the quantity of the latter, 

 so that an alloy was precipitated, which was continually increasing the copper with 

 respect to the gold, till he had a speculum whose surface was gold, and which then be- 

 came an alloy, the quality decreasing, till, at the bottom,it became pure copper. This 

 was important; because, without such experiments, one would not have known that 

 such results would have followed; for some philosophers supposed, that, if we attempt 

 to precipitate the salts of two metals, only one is precipitated ; but Prof. Steinheil 

 informed him that they precipitated in union. He thus obtained a speculum with a 

 face of gold and a back of copper. 



But, supposing the largest, cheapest, and best speculum was obtained, the framework 

 of the telescope would be so gigantic, that few observers would be able to use the in- 

 strument. With a focal length of sixty to eighty feet, it would be quite unmanageable 

 for any private individual. The idea occurred to him (Mr. Talbot), to have a tube 

 fixed in an invariable position, and to have a perfectly true plane mirror, of a size 

 somewhat larger than the concave speculum, placed in front of the tube, with an aper- 

 ture in the centre. This plane reflector should be moveable about its centre in any 

 direction ; so that rays from luminous bodies, falling first upon the plane reflector, were 

 then reflected against the concave reflector, and then passed through the aperture. The 

 only motion requisite for the plane mirror would be one about its centre. The me- 

 chanical difficulties in the way of this plan would be far less than in the common method. 

 Prof. Steinheil's idea on this point was somewhat different. He (Mr. Talbot) had 

 thought of placing the tube in a horizontal position, which, in the case of a tube of 

 very great length, has manifest advantages. Prof. Steinheil's idea was, that it should 

 be pointed directly to the pole of the heavens. 



On the Theory of Magnetism. By John Goodman. 



Having frequently attempted in vain to produce electro-magnetic effects by the 

 current from the electrical machine and metallic conductors, the author at length 

 resolved upon employing such imperfect conductors of the current as would arrest 

 the speed of the frictional fluid, and cause it to progress more in the manner of vol- 

 taic electricity. For this purpose he used a slip of common writing-paper as a con- 

 ductor in place of copper wire employed in electro-magnetism, and instead of the 

 soft iron substituted a plate of thin window-glass. This plate being arranged upon 

 insulating pillars, it was found, after several test experiments, that the greatest 

 weight that could be sustained was supported by suspending the same to a card or 

 paper armature affixed to the under surface of the glass, whilst a continuous current 

 from the positive to the negative conductor of the machine was transmitted along 

 the upper surface of the plate by means of two half sheets of writing-paper. _ In 

 this manner a weight of five ounces and twenty grains was sustained for some time, 

 which, in proportion to the quantity of electricity in the current, is probably as great 

 ■as is ever supported by any electro-magnet whatever. 



From the success of this experiment the author contended, that for anything at 

 present known this may be the principle of magnetic action ; nor does he see any 

 reason for seeking other explanation of magnetic phsenomena than the simple laws 

 of ordinary electric attraction and repulsion. He also suggested that the crystalline 

 nature of iron may render it more capable of polarization by electric influence than 

 the other metals ; and if a polar effect be obtained, so that the electricity of one par- 



1842. c 



