The formation of an image by means of a concave mirror is exhibited in fig.3.As the observer's head would be placed between the object and the mirror, if the image, formed as in fig.

3, were to be microscopically examined, various devices are employed in the construction of reflecting telescopes to avoid the loss of light which would result--a loss which would be important even with the largest mirrors yet constructed.

Thus, in Gregory's Telescope, a small mirror, having its concavity towards the great one, is placed in the axis of the tube and forms an image which is viewed through an aperture in the middle of the great mirror.

A similar plan is adopted in Cassegrain's Telescope, a small convex mirror replacing the concave one.

In Newton's Telescope a small inclined-plane reflector is used, which sends the pencil of light off at right-angles to the axis of the tube.