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that the images formed by them are necessarily a little imperfect: now in Gregory's telescope, the two mirrors correct each other if they are properly matched. For this reason a careful optician always tries several small mirrors and chuses the best.
Cassegrain's Telescope. Fig. 193.
175. This bears the same relation to Gregory's that Galileo's does to the astronomical telescope; the small speculum is placed between the large one and its focus, and is convex, so that the second image is thrown near the eye glass as before: this image is, however, inverted.
This instrument is of course shorter than Gregory's, and it appears, in theory at least, to have a considerable advantage over it in that the spherical aberrations of the two reflectors tend to correct each other,[1] and the second image should therefore be more perfect; this is, however, not discernible in practice, and for some reason or other the construction is seldom used.
The magnifying power is here (l+F′)2f′F.
176. Dr. Brewster, in the Edinburgh Quarterly Journal for October, 1822, suggests a construction for a reflecting telescope something like that of Sir Isaac Newton. He proposes to substitute for his plane mirror, or reflecting prism, an achromatic pair of prisms which should divert the image in an oblique direction towards one side of the instrument where the eye glass should be placed so as to view it directly, (Fig. 194.).
He also suggests the idea of dividing the converging light among three or four such prisms which should convey images to different parts of the instrument where they might be viewed at once by several different observers.
Mr. Airey of Trinity College has invented a new reflecting telescope, in which lenses, silvered on one side, are substituted for
- ↑ The aberration in the first reflection is from the center, in the second towards it, and both the centers lie on the same side of the foci.
