of different Species of Glass. £93 
The distances of some of these coloured points, for example 
the violets, the blues and the reds, whose light is weak, cannot 
be measured without illuminating the micrometer wires of the 
telescope. These coloured points, however, lose, by the ordinary 
method of illuminating the field, as much light as the wires re- 
ceive, and therefore this method cannot be employed. It was ne- 
cessary therefore to have a mechanism by which the wires alone 
could be illuminated, while the rest of the field remained dark. 
Such a mechanism I applied to my micrometer. The illumina- 
ting of the wires may thus be modified at pleasure, and always 
with facility. This is effected on the side of the eye-glass by 
means of a small lamp inclosed in a hollow globe, from which 
the light falls upon a lens, and throws it in a parallel manner 
on the wires. At the inner margin of the eye-glass, construct- 
ed for the purpose, the rest of the incident light is absorbed 
without falling on the lens. 
With this apparatus I have measured the angles of refrac- 
tion of the different coloured rays for several refracting sub- 
stances, the results of which are given in the following Table. 
With all the substances, the angle of the incident ray is equal 
to the angle of the emergent ray N. Each angle was measured 
four times. Since the light which sets out from A does not fall 
in a parallel manner on the prism H ; or rather, since the plane 
in which the prism II is placed is not in the axis of the theodo- 
lite, but at its centre is distant from the axis 4.25 inches, it 
was necessary to apply a small correction to the angle that the 
incident ray makes with the emergent ray N. The distance of 
A from II being 692 feet, the correction for the prism of flint- 
glass is -J- 31", for that of crown-glas 40", and for water 40 // , 
he. The angles LM, MM, he. do not require this correc- 
tion. 
