A. W. Wright—Electrical Deposition of Metals. 173 
through them. ‘he thickness of the gold layer was found to 
be 0:000183 mm., that of the platinum 0°000174 mm., or 
+ pore: one-fourth the length of a wave of light at the 
red end of the spectrum. The gold, although thicker than the 
platinum, transmits perceptibly more light, showing that it is 
the more transparent of the two metals. As the films employed 
for mirrors may be much thinner than the amount mentioned 
possible to apply the method of local correction for the im- 
provement of a defective figure, or to parabolize a spherical 
mirror by depositing the metal in a layer increasing in thick- 
ness toward the center, though, of course, it would be better to 
avoid a somewhat tedious operation by securing the perfect 
form of the glass beforehand. — 
Of the metals that are suitable for the formation of specula, 
platinum appears to be the most valuable. For while, when 
well polished, it is but little inferior to silver in reflecting 
power and freedom from color, it does not become tarnished by 
oxidation or the action of sulphurous gases, and when dulled 
by atmospheric deposits the surface can be cleaned by washing 
with water or with acids, which is an important advantage. 
By the method here described it can be deposited upon glass 
