Stone Y — On a New Form of SjicctroHcopc. 
lei beam, it is by it brought to a focus on 
a, by whicli it is reflected into the eye- 
piece E. The image being formed on the 
back of a, the eye-piece must be a micro- 
scope capable of examining an object at 
that distance. The semi-prism will be 
filled with bisulphide of carbon, and will 
be plunged in a tank containing a liquid 
which shall possess the three following 
properties : a low dispersive power, a re- 
fractive index bordering upon that of the 
glass window of the semi-prism, and a 
specific gravity nearly that of bisulphide 
of carbon. We are at present engaged in 
ascertaining whether glycerine or a solu- 
tion of zinc chloride most nearly combines 
the desired qualities. The tank is de- 
signed to discharge three useful functions ; 
to prevent a straining pressure upon the 
window and mirror of the semi-prism, to 
diminish the deviation of the light, and to 
render changes of temperature in the bi- 
sulphide of carbon slow. The mechanical 
part of the arrangement is of the simplest 
kind. A motion of the prism round a 
vertical axis will bring all parts of the 
spectrum under review. In the field of 
view will be seen both the spectrum and 
the patch of the back of a, to which /3 is 
cemented. /2 should extend a little more 
than half way along the height of u,, and 
then by very slightly tilting the mirror Jf, 
the vacant patch, and the lines of the 
spectrum will be seen as in the adjoining 
woodcut. 
The diameter of the mirror is to be 
twenty-one centimetres, and the other 
parts in proportion. This gives forty-two 
as the standard power, and we expect to 
obtain with it a dispersion of between 250 
