90 Phenomena of Polarized Light. 
and in addition a charming system of 
aye rbolas. The pieces, each about 
th of an inch thick, are cemented 
into frames revolving one within the 
touching. In viewing this figure by 
polarized light, the Nicols prism is 
held perpendicular to the mica plates, 
and not inclined as in viewing a sin- 
gle thickness of mica. Fig. lisa rep- 
resentation of this interesting compo- 
site system. ‘The colors are brilliant 
and the negative figure is not less pleasing than the positive. 
Polarization by Caoutchouc and Gutta Percha.—Gutta per- 
cha when rolled into thin sheets or drawn into ropes comports 
itself like a fibrous substance, which is not the case with caout- 
chouc. A strip cut from a thin sheet of gutta percha may be 
stretched considerably in one direction, that is, in a line with the 
fibre, but any attempt to stretch it across this line, is followed at 
once byarupture. It isnot so with a sheet o tchouc, which 
will stretch equally well in all directions. Ga. pi ia of 
thin sheets of these two substances—so far believed to be is0- 
erical—a marked difference of texture is at once perceived. 
The caoutchouc gives little or no change of color, while the gut- 
ta percha exhibits a beautiful spectacle. It appears to be built up 
of prisms of every variety of hue, and as it were fused into each 
other. It resembles more nearly some specimens of ice which 
have examined, than any thing else. The caoutchouc and gutta 
percha must be kept under considerable tension during the exam- 
ination. 
seit is one mode however, in which I have produced some 
res by means of caoutchouc. The caoutchouc is made 
into Lite balloons in the parece manner. Avery thin sheet is_ 
tied over the end of a tube of one half inch bore, and the caout- 
chouc blown out into a ball and firmly tied just beets the end 
of the tube with a piece of silk. The form 
and size of these balloons are nearly as repre- 
sented by fig. 2. They can with a little prac- 
tice be very conveniently made by the mouth. 
The caoutchouc is drawn over the open mouth 
and by strong suction it is forced in filling the 
mouth, when the lips and teeth are compress- 
ed over the outer portions, so as to form a neck 
which is at ‘gan ee up by the fingers and 
secured with a The caoutchouc thus 
