On the same principle as that by which a refracting telescope may be constructed by means of a single lens—as represented fig. 51, (page 234) we may form a telescope by reflection with a single mirror, and without an eye-piece. Let AB, fig. 72, represent a large concave speculum, and C its focus—if an eye be placed at D, about 8 or 10 inches within the focal point C, all the objects in the direction of C, or behind the spectator, will be seen magnified by reflection on the face of the mirror, and strongly illuminated. The magnifying power, in this case, will be nearly in the proportion of the focal length of the mirror to the focal length of the eye for near objects. If for example, the focal distance of the mirror be 8 feet, and the distance from the eye at which we see near objects most distinctly, be 8 inches—the magnifying power will be in the ratio of 8 to 96, or 12 times. I have a glass mirror of this description, whose focal length is 4 feet 8 inches, and diameter 6 inches, which magnifies distant objects about 7 times, takes in a large field of view, and exhibits objects with great brilliancy. It presents a very distinct picture of the moon, showing the different streaks of light and shade upon her surface; and, in some cases, shows the larger spots which traverse the solar disc. This mode of viewing objects is extremely easy and pleasant, especially when the mirror is of a large diameter; and the observer is at first struck and gratified with the novel aspect in which the objects appear.

Were a concave mirror of this description—whether of glass or of speculum metal—to be formed to a very long focus, the magnifying power would be considerable. One of 50 feet focal length, and of a corresponding diameter, might produce a magnifying power, to certain eyes, of about 75 times; and, from the quantity of light with which the object would be seen, its effect would be much greater than the same power applied to a common telescope. Sir W. Herschel states, that, on one occasion, by looking with his naked eye on the speculum of his 40 feet Reflector, without the interposition of any lens or mirror, he perceived distinctly one of the satellites of Saturn, which requires the application of a considerable power to be seen by an ordinary telescope. Such an instrument is one of the most simple forms of a telescope, and would exhibit a brilliant and interesting view of the moon, or of terrestrial objects.

PRICES OF REFLECTING TELESCOPES.

1. Prices as stated by Messrs. W. and S. Jones, Holborn, London.

£s.
A 4 feet, 7 inch aperture Gregorian reflector, with the vertical motions upon a new invented principle, as well as apparatus to render the tube more steady for observation, according to the additional apparatus of small speculums, eye-pieces, micrometers, &c.from 80l. to 1200
Three feet long, mounted on a plain brass stand232
Ditto with rack-work motions, improved mountings and metals3918
Two feet long without rack-work, and with 4 magnifying powers, improved1515
Ditto improved, with rack-work motions221
Eighteen inch, on a plain stand99
Twelve inch ditto66

2. Prices as stated by Messrs. Tulley, Islington.

£s.
1 foot Gregorian Reflector, on pillar-and-claw stand, metal 2½ inches diameter, packed in a mahogany box66
1½ foot ditto on pillar and claw stand, metal 3 inches diameter, packed in a mahogany box1111
2 feet ditto, metal 4 inches diameter1616
Ditto with rack-work motions254
3 feet ditto, metal 5 inches diameter, rack-work motions420
4 feet ditto, metal 7 inches diameter, on a tripod stand with centre of gravity motion1050
6 feet ditto, metal 9 inches diameter2100
7 feet Newtonian, 6 inches aperture1050
12 feet ditto, metal 12 inches diameter5250

3. Prices stated by Mr. G. Dollond, St. Paul’s Church Yard.

£s.
Reflecting telescopes 14 inches long, in a mahogany box99
Ditto, 18 inches1212
Ditto 2 feet1818
Ditto with 4 different powers, and rack-work stand supporting the telescope in the centre of gravity3615
Ditto 3 feet, with ditto500