1881.] Some Experiments on Metallic Reflexion. 



491 



Polishing the plate with rouge appears to have had the effect of 

 increasing the values of the principal incidences and diminishing those 

 of the azimuths, but not to such an extent as might have been antici- 

 pated from the difference between the values obtained with the plate 

 before and after it had been polished with putty powder. This may 

 be due to the original smoothing and polishing with rouge having 

 altered the surface of the plate to a greater extent than could be 

 effected by repolishing with rouge the already polished plate. 



The gold plate, which was a long narrow one, having been con- 

 stantly rubbed in one direction whilst being polished, it was thought 

 possible that the direction of the furrows produced by the polishing 

 might influence the result: the plate was therefore turned so that 

 these were perpendicular to the plane of incidence, instead of parallel 

 with it. This alteration in the position of the plate did not appear to 

 alter the value of the constants. 



Eed light. 

 P.I. P.A. 



r Plate at A 76 42 40 44 



Gold in air. . } Plate at B 72 23 39 54 



L Mean value 74 32 40 19 



In order to obtain a gold surface free from the influence of any 

 polishing powder, a sheet of gold leaf was placed between two pieces 

 of paper, and. the paper and gold cut into strips with a sharp pair of 

 scissors, the gold leaf transferred to glass slips, floated out with water, 

 in the manner described by Faraday, and the water allowed to drain 

 off, leaving the gold leaf stretched out on the glass. 



Some of the pieces of gold leaf were thinned by being floated on a 

 dilute solution of potassium cyanide, the cyanide solution washed 

 away with water as soon as the thickness of the gold had been suffi- 

 ciently reduced, and the gold leaf left adhering to the glass. 



Gold leaf treated in this way has a very fairly smooth surface, 

 sufficiently good to act as a tolerable mirror, and both the ordinary 

 leaf, and that thinned by the action of potassium cyanide, is trans- 

 parent, the transmitted light being, as is well known, green. 



In order to determine the thickness of the gold, the glass slips were 

 carefully cleaned and weighed before the gold was transferred to them, 

 and then, after being dried, again weighed. 



The area of the gold was measured, and the specific gravity being 

 taken as 19'36 ("Watts' Dictionary," vol. ii, p. 926), the thickness 

 calculated. 



The average thickness of gold leaf is stated in "Watts' Dictionary" 

 (loc. cit.) not to exceed ^ooVoo °f an i ncn - ('0001270 millim.), and in 

 " Roscoe and Schorlemmer's Chemistry " to be about '0001 millim., 



