TELESCOPE. Such telescopes as are onstructed with glass lenses only are called tioptric, or refracting telescopes ; whereas hose which have a reflecting speculum are he catoptric, or reflecting telescopes. By these nstruments objects even in the remotest lepths of space are rendered accessible to Inman vision ; and terrestrial objects faintly risible in the distance are brought as it were lose to the eye. There is evidence to show hat Roger Bacon, who died in 1292, knew hat lenses might be so combined as to make ibjects seen through them appear to be mag dfied. No further indications of telescopes ire met with till after the middle of the 10th entury, when Dr. Dee suggested that the ommander of an army, who wants to dis over the number and arrangement of an nemy's troops, 'may wonderfully help him elf by perspective glasses.' In 1609 Galileo onstructed a telescope, with which he dis overed the four satellites of the plane; Jupiter. Huygens, Dr. James Gregory, Sir Isaac Newton, Dr. Hooke, Dr. Bradley, Sir Wm. Herschel, Mr. Dollond, and Lord Rosse, have since been the persons most dis• tingnishecl for the improvements which they have respectively made in the construction of telescopes: To explain the optical principles which govern the action of telescopes would be beyond the scope of the present volume; but we may briefly explain differences of con struction.
The telescope invented by Galileo consisted of one convex lens and one concave lens ; the distance between them being equal to the difference between the focal lengths of the two lenses. This is the construction of what is called an Opera Glass; and the Galilean telescope is now used chiefly for viewing objects within a theatre, or an apartment ; since if considerable magnifying power were given to it the extent of the field of view would be very small.
A simple telescope may also be constructed by means of two convex lenses, which are placed at a distance from one another equal to the sum of their focal lengths.
In order to afford a view of objects in the same position as they appear to have when seen by the naked eye, Mr. Dollond employed an eye-tube containing four lenses ; whereas in the eye-piece invented by Huyghens, which is used in most astronomical telescopes, there are only two lenses, and objects are seen inverted.
In reflecting telescopes, a speculum at one extremity of the tube serves the purpose of the object-glass in refracting telescopes by forming an image at its focus ; and the image so formed is viewed by the eye through inter mediate reflectors. The Newtonian reflecting telescopes have one concave speculum at the bottom of the tube ; and the rays reflected from it fall in a convergent state upon a small plane mirror placed so as to make an angle of with the axis of the telescope : after the second reflection the rays unite and form an image which is viewed through a Huygenian eyepiece fixed in the side of the tube, oppo site the plane mirror; that is, open end of the tube. In the Gregorian reflecting telescope the second reflection is given by a second concave mirror, the face of which is towards the observer.
The telescope constructed by the late Sir Wm. Herschel differed from the Newtonian telescope only in having no small mirror. The surface of the great speculum, which was 4 feet in diameter, had a small obliquity to the. axis, so. that the image formed by re fiection from it fell near the lower side of the tube at its open end : at this place there was a sliding apparatus which carried a tube con taining the eye-glasses. The observer in viewing was situated at the open end of the tube, with his back to the object, and he looked directly towards the centre of the speculum. The reflecting telescope executed by Lord Rosse in 1842 is 50 feet long, and its speculum is 6 feet in diameter. It is capable of being directed from the zenith to the horizon towards the south, and from the zenith to a position parallel to the earth's axis towards the north ; it has also a move ment in azimuth of about 8 degrees on each side of the meridian.
The Great Exhibition contains a noble telescope by Mr. Ross, which is, we believe, the largest ever constructed on the refractive principle.