TRANSACTIONS OF THE SECTIONS. 13 



spect of obtainiDg a material for the plain speculum more reflective than the alloy 

 of tin and copper, no time nor labour could be misapplied in endeavouring to effect 

 so important an object. It was not until Jamia's ' Menioire sur la Couleur des 

 Metaux' appeared in the ' Annalesde Chimie ' for 1848, that I was aware that silver 

 reflected so very much more light than speculum metal. Jaaiin states that he has 

 abundantlvverified Cauchy's formulae for the laws of metallic polarization ; £md from 

 these laws, by the aid of certain constants, he has determined the intensity of light 

 reflected by some of the metals. From the table he has given, it appears that while 

 speculum metal reflects about sixty rays out of a hundred, sih^er reflects ninety. 

 Not from having any doubt of the accuracy of Jamin's deductions, but happening to 

 have the means at hand, I made a few coarse photometric measurements of the re- 

 lative reflective powers of silver and speculum metal, and the results appeared to 

 coincide sufficiently with the more accurate deductions of Jamin. Not feeling very 

 sanguine as to the practicability of procuring an accurate surface of silver by me- 

 chanical means, owing to its softness, I tried the electrotype process in the first in- 

 stance. The silver was thrown down upon a surface of highly polished speculum 

 metal ; but in every case where the speculum metal was thoroughly clean there was 

 strong adhesion, so that separation could not be effected without destroying both 

 surfaces. The means which were employed to guard against adhesion in electro- 

 typing the engraved copper plates for the Ordnance Map of Ireland, which, in fact, 

 consisted in applpng an extremely thin film of wax, would obviously be inadmissible 

 in this case. Several attempts to precipitate silver on a steel speculum failed, as the 

 silver had not a proper polish. Being however without experience in electrotyping, 

 I do not consider these failures conclusive. Attempts were made with a steel die 

 truly polished to procure accurate polish by pressure. This did not succeed : the 

 surface was not sufficiently true, owing apparently to unequal elasticity in the tex- 

 ture of the silver. To turn or plane a polished surface was not attempted, as the 

 idea had suggested itself to the late Mr. Barton, and if it had been practicable in 

 his hands it would doubtless have succeeded. There remained therefore but to try 

 some modification of the ordinary processes of polishing metals. A difficulty oc- 

 curred in the beginning, which gave considerable trouble. Owing to the softness of 

 the silver, the emery employed to grind it flat became imbedded in it, and in that 

 state to polish it was quite impossible. Without grinding, however, there seemed 

 to be no means of making the silver sufficiently flat, as Whitworth's scraping process 

 had been tried, and was not found to be sufficiently delicate. The difficulty was 

 obviated by employing a bed of German hones, by which the silver is probably 

 rather filed than ground. The blue hones might perhaps be employed with advan- 

 tage after the German hones, but I have not tried them. The silver being thus pre- 

 pared, I tried the ordinary process of polishing on pitch in vain, employing the 

 pitch of various degrees of hardness : the surface of the silver was irregular and the 

 polish imperfect. The silversmith makes use of chamois leather and rouge, some- 

 times finishing with the hand charged with rouge : his polish is very brilliant, but 

 the surface is, as we should expect, very untrue, as for example, the surface of a 

 highlv finished plateau. It was rather puzzling to find that while chamois leather 

 charged with rouge polished silver, pitch, however soft, did not : there was no ap- 

 parent difference in the two cases, but this ; that the pitch slightly shielded by the 

 rouge came more or less in contact with the silver, while the chamois leather, hold- 

 ing fast the stratum of rouge, was scarcely in contact at all with the silver. That 

 this was the true reason was probable, as in proportion as the chamois leather was 

 less shielded, the process was imperfect. It is evident that, however fine the polish, 

 a true surface could not be procured by the action of an elastic material, and there- 

 fore pitch was taken as a basis ; a substance which is solid, and at the same time 

 adapts itself in the most perfect manner to the surface to be polished. We pro- 

 ceeded in this way : — Pitch of the proper hardness for polishing speculum metalwas 

 covered with a mixture of rouge and the combination of ammonia and soap which 

 we employ in polishing specula. The silver was worked upon this for a short time 

 to force the rouge into the pitch, after which the rouge mixture was again applied 

 and suffered to dry. The surface w£i3 then slightly moistened with spirits of tur- 

 pentine, and more of the rouge mixture applied. The following day, the turpentine 

 having evaporated, there was a rouge surface, perhaps of xicsth of an inch thickness. 



