Chemistry and Physics. * 111 
accompanying figures. The rays from the two sides of the object glass 
are transmitted through a compound achromatic prism shown in fig. 1 
by which they are made to cross each other and diverge at an angle of 
ence after leaving the prism. In an instrument of 
this kind, made under the direction of the writer, by 
Messrs. Grunow, the flint prisms have each a refrac- 
ting angle of 26°, the lateral anglés of the crown 
prism are 44°; the opposite faces of the crown 
prism are parallel; the refractive index of the flint 
glass is 1:621 and of the crown glass 1°534 and the 
dispersive powers of the flint and crown prisms are as 1-000 :0°625. 
n hg. 2, @ represents the object-glass of the microscope, 8 is the com- 
pound prism, ¢¢, ¢¢ show the position and direction of the two lateral 
pencils, and dd are two eye-pieces at the ex- == 
tremities of the two branches of the compound 
y and the microscope. 
between the centres of the eye-pieces is 2 inches, 
and when drawn out two inches, the distance 
apart is 2} inches. This is as great a range as 
1s required to suit the eyes of most persons who 
Wish to use the microscope. 
A great superiority of this binocular micro- 
Scope over other forms, as Nachet’s and Rid 
dell’s, results from the extreme thinness of the 
chromatic prism. In both of the other forms 
named, the light, twice suffers total reflection, 
and passes through a thickness of an inch or 
more of glass causing much aberration and loss 
