Page:Astronomy for Everybody.djvu/92

The practical difficulties in using a reflector are several in number. The first and most obvious one is that the rays are reflected back in the direction from which they came. To see the image the observer must look into the mirror as it were. If he does this directly, his head and shoulders will cut off the light that falls on at least the central regions of the mirror. Some contrivance for reflecting this light away is therefore necessary. Two ways of doing this are in use. In what is known as the Cassegranian reflector, a smaller, slightly convex mirror is interposed between the focus and the principal mirror. An opening is made in the centre of the latter, through which the rays are reflected back by the smaller mirror. The curvature and positions of the two are so adjusted that the image of the distant object shall be formed in this opening. The only telescope of this kind in actual use is the great Melbourne reflector, of four feet diameter, made by Sir Howard Grubb, of Dublin.

The contrivance most in use was designed by Sir Isaac Newton. It consists of a diagonal reflector, which may be a mere glass prism, placed just inside the focus. Its reflecting surface makes an angle of forty-five degrees with the axis of the telescope, and therefore reflects the rays laterally to the side of the tube. Here they are observed with an ordinary eyepiece. This instrument is known as the Newtonian reflector.

It is remarkable that, notwithstanding the immense improvement in the mechanical processes necessary in constructing and mounting a reflecting telescope, no attempt has ever been made to even equal Lord Rosse's