Br Brev;ster on a New. Reflecting telescope. 
As Dr Maskelyne found that the Greenwich Newtonian tele- 
scope was greatly improved by inclining the speculum, or rather 
its axis, about S|°.to the axis of the tube, it follows, that it per- 
formed best with oblique pencils in a certain direction. Before 
fitting up this telescope, therefore, or indeed any Newtonian te-^ 
lescope, it should be ascertained, by turning the speculum in its 
cell, and giving its axis various degrees of obliquity, whether or 
not its performance is improved. If this is found to be the 
case, and that the angle of inclination is, for example, we 
shall then require a less angle in the prism G, as part of the 
deviation of the pencil to the side of the tube is already produ- 
ced by the new position of the axis of the speculum. 
It is obvious, that any rays incident on the prism GH, paral- 
lel to the tube, will be refracted upon the side of the tube, and 
wiU have no effect upon the image; but if they should be thought 
injurious, we have only to place an opaque screen, of the same 
size as GH and its arm, somewhere between GH and the mouth 
of the tube, so as not to interfere with the cone of refracted rays 
GH^ 
This form of the reflecting telescope enables us to construct 
a very simple double image micrometer, the prism H, either 
when united with G, as in Fig. 1. or when separated as in 
Fig. 2. being made of a doubly refracting crystal, cut in such 
a manner as both to correct the colour of G, and produce the 
two images. 
In viewing eclipses of the sun, or the lunar disc, or any other 
celestial phenomenon, where there is either too much light, or 
more than is necessary, a telescope may be fitted up to permit 
more than one person to see through it at the same time, by al- 
lowing a portion of the cone of rays to be refracted through an- 
other prism to an eye-glass at the opposite side of the tube, or 
even by having four prisms refracting a fourth part of the cone 
to each of the four sides of the tube. The same effect might be 
produced in the common Newtonian telescope, by using a pyra- 
midal small speculum, with the planes meeting at a vertex placed 
in the centre of the cone of rays, and inclined to one another at 
angles of 90°, and to the axis of the telescope at angles of 45°. 
Another person at A might also be made to see the eclipse by 
means of a parallel plate of glass at m inclined so as to re- 
