"386 Royal Society. 
some difference appeared: the polarization of light by mother 
of pearl is different from that of the bedies which he had before 
examined : he found it impossible to give wax, cement, &c. ex- 
actly the same powers as Iceland spar, although they receive the 
impression from mother of pearl, so as to produce similar optical 
phenomena. It appears that the particular laminous structure 
of mother of pearl is such, that, however finely it may be polished, 
it still retains the power of reflecting highly coloured images, 
and that if a piece be merely cut it reflects only one image, but 
if polished two are seen. These facts are ascribed to the pecu- 
liar striz of this substance, which are very fine, yet may in some 
specimens be seen with the naked eye; in others there are above 
3000 of them in an inch. 
May 5. Capt. Cater furnished a long paper, containing a 
description of a new instrument for dividing mathematical in- 
struments, which he considers an improvement of Troughton’s ; 
he acknowledged that he was anticipated in the principles of his 
improvement, but alleged that it was never before reduced to 
practice. A brief description of this instrument would not be 
intelligible without a plate: to it he applies three microscopes, 
one of which is fixed and two are moveable, and suggests modes 
of correcting the errors of observation, and preventing the mi- 
croscopes from coming in contact with each other. 
May 12. Dr. B. Heyne reada paper to the Society, relating 
the process by which the Hindoos oxidate silver for medical 
purposes. The secret was communicated to him by an ancient 
and learned Hindoo, by whom it is esteemed of great value; he 
afterwards repeated the process himself. A rupee which weighs 
three drachms was hammered into a plate three inches broad ; 
it was then immersed in the milk of plants, chiefly of the ew- 
phorlia genus. The plate was heated, and plunged into this fluid 
above twenty times, when it acquired only a dark-gray colour ; 
it was afterwards placed between the leaves of plants, and re- 
peatedly heated to a degree below smelting, and each time 
plunged into the milk of plants or cow-dung, till the plate was 
finally oxidated, so that it could be squeezed to powder between 
the fingers ; and in this state it is given as a sovereign remedy in 
most diseases. Dr. H. found that water produced the same 
effect in facilitating the oxidation, as the milk of plants or cow- 
dung. As he could find no information in: chemical works re- 
specting the particular properties of the milk of plants, he was 
induced to make some experiments on that from several species 
of euphorbia. In some the. juice is aptly called milk, like the 
caoutchouc or elastic gum ; in others it is colourless or thin, and 
inspissates when exposed to the air; others again emit a deep- 
coloured . fluid, which assumes a gummy or resinous character. 
In 
