TEL 



Ifi3 



TEL 



The great telescope constructed by Herschel differs 

 from the Newtonian telescope only in having no small 

 mirror. The surface of the great speculum, which is 4 

 feet in diameter, has a small obliquity to the axis XY, so 

 that the image formed by reflection from it falls near the 

 lower side of the tube at its open end: nt this place there 

 is a sliding apparatus which carries a tube containing the 

 eye-glasses. The observer in viewing, is situated at the 

 open end of the tube, with his back to the object, and he 

 looks directly towards the centre of the speculum, the 

 magnitude of which is such that the rays intercepted by 

 his head, in coming from the object, do not in any sensible 

 degree diminish the brightness of the image. 



Formerly the great speculum of a reflecting telescope 

 was pressed into its cell by means of springs attached to 

 the interior side of the brass plate at A: but the vibra- 

 tions of the spring* were found to cause tremulous mo- 

 tions in the image at the focus of the mirror; and this 

 (.fleet was so great as to render reflecting telescopes in- 

 ferior to those of the dioptric kind. The Reverend Mr. 

 Edwards, who detected the cause of the tremors, at once 

 removed it by taking away the springs (Ncut.Alm.. 1787); 

 and the same gentleman further improved the distinctness 

 of the image by enlarging the aperture to which the eye 

 is applied. It lias been observed also that when the 

 great speculum is nearly in a vertical position, and con- 

 sequently rests on its lower extremity, its weight bends it, 

 nnd thus causes a change in the figure of its polished sur- 

 face : on this account it is recommended thnt the specu- 

 lum should be made to rest on two small wedges, placed 

 one on each side, at about 45 degrees from the lowest 

 point. 



lifsides the power of magnifying objects, that of afford- 

 ing distinct vision with given quantities of light is often 

 tntiul riMuiiute in a telescope, particularly to navnl 

 men, who have occasion during the obscurity of the night 

 to keep in view a ship of which they may be in chase. 

 This subject was investigated by the late Sir William 

 Herschel, and an account of his researches on what 1m 

 calls the 'space-penetrating power of telescopes' was 

 printed in the I'hilnMi^hii-nl Tninxurtioiix/nr 1S()0. 



Herschel states that he was aware of this property of 

 >pes as early as the year 1777. when he had con- 

 structed a Newtonian telescope with a speculum whose 

 focal length was 20 feet : for, on directing th" instrument 

 to a church-steeple at a considerable distance, he could 

 distinguish the hour by the clock, though with the naked 

 eye he could not see th'e steeple itself. In order to obtain 

 a formula for the space-penetrating power, he observes that 

 the quantity of light received by the natural eye varies 

 directly with the aperture of the "pupil, or with tlip square 

 of its radius, and inversely with the square of the distance 

 of the object: also that the quantity of light transmitted 

 by a telescope, supposing none to be lost in the reflections 

 trom the mirrors, or in refraction through the lenses, will 

 vary directly with the square of the radius of the aperture 

 and inversely with the square of the distance of the < 

 But, from experiments on the measure of light, it appears 

 that the whole quantity incident upon a plate of glan m 

 to the quantity transmitted through it as 1 is to <);{ I. or 

 to the quantity lost as 1 is to -(.Mil!) ; and from this, the whole 

 quantity of incident light being unity, an estimate may be 

 made of the quantity of light transmitted through all' the 

 lenses of a telescope : with respect to the quantity lost in 

 reflection from mirrors Sir W. Herschel found that out of 

 1(10,000 incident rays, only 45,242 reached the eye alter two 

 reflections. 



Let the quantity of incident light be to that which 

 arrives at the eye as 1 to m ; then / beiuir the radius of 



Df 



the pupil, and R that of the aperture of a telescope, 



will express the ratio between the quantity of light trans- 

 mitted to the. naked eye, and through a dioptric telescope: 

 therefore the space-penetrating power varying with the 



R 



square root of the quantity of light, V' expresses the 



penetrating power. With respect to reflecting telescopes, 

 n K' be the radius of the small speculum, the pern- 



powerwill be expressed by i,/{m(R-R)}. It is neces- 

 sary fc observe that, in these expressions, it w supposed 



that the pencil of light transmitted by the telescope is not 

 greater than the pupil of the eye. 



TELESCOPE, HISTORY OF THE. It has been the 

 fate of almost every instrument by which science has been 

 extended, or the well-being of man promoted, that the 

 precise epoch of its invention, and even the name of the 

 individual to whom the world is indebted for it, are alike 

 unknown. This is particularly the case with the telescope, 

 of which the earliest notices are that it existed in England 

 ami in Holland near the end of the sixteenth or in the 

 beginning of the seventeenth century. 



There is in Strabo a passage (hi., p. 180, Falconer's ed. ; 

 p. U8. Casaub.) in which, speaking of the enlargement of 

 the sun's disk at his rising and setting in the sea. it is stated 

 that the rays (of light") in passing through the vapours 

 which rise from the water, as through tubes, are dilated, 

 and thus cause the apparent to be greater than the real 

 magnitude (of the object) ; and from this it has been in- 

 terred (Dutens. Rfehtrrhm. &e.\ though the inference is 

 probably without foundation, that there" then existed tubes 

 furnished with lenses for magnifying objects by refracting 

 the light. It would be needless to make any observa- 

 tions on an inference founded upon an hypothesis so 

 obscurely expressed : the words in Strabo probably sig- 

 nify only that the rays of light might become divergent 

 in passing along the intervals between the particles of 

 vapour. 



Omitting then all notice of this, and of the ill under- 

 stood passages in Aristophanes ('Clouds') and Pliny (lib. 

 xxxvi., c. 67) concerning transparent spheres, or lenses for 

 concentrating the rays of light, it must be acknowledged 

 that before the end of the thirteenth century lenses of 

 glass were in constant use for the purpose of assisting the 

 eye in obtaining distinctness of vision. Vitello, a native 

 of Poland, in that century, gave some obscure indications 

 of the apparent magnitudes of objects when viewed through 

 nent of a sphere of glass: and Roger Bacon, in his 

 Opus Majus.' both mentions the like fact, and expresses 

 himself in such a manner as to indicate that in his time 

 (he died in 12021 spectacles were already in use. He may 

 not have actually made combinations of lenses in one in- 

 strument, but there is no doubt of his being aware of the 

 tact that lenses might be arranged so that objects seen 

 through them would appear to be magnified. [BACON, 

 KooKit : SPKCTACLKS.] 



The idea being known to the learned, it is scarcely pos- 

 sible to doubt that the combination of two lenses, or of a 

 concave or convex mirror and a lens, must have been often 

 made during the three centuries which elapsed between 

 the time of Bacon and that which is generally considered 

 as the epoch of the invention of telescopes. Dr. Dee, in 

 his preface to Euclid's 'Elements' i 1570 1, having men- 

 tioned that some skill is required to ascertain the numeri- 

 cal strength of an enemy's force when at a distance, ob- 

 serves that the ' captain, "or whosoever is careful to come 

 near the truth herein, besides the judgment of his eye, the 

 help of his geometrical instrument, ring, or static astrono- 

 mical (probably for determining the measure of dis- 

 tances . may wonderfully help himself bv perspective 

 glasses:' these last can only signify some kind of telescope, 

 which therefore must have been then in genera! 

 And in the ' Pantometria ' of Digges ( l.T/l ) it is stated 

 that ' by concave and convex mirrors of circular 'spherical) 

 and parabolic forms, or by frames of them placed at due 

 angles, and using the aid of transparent glasses which 

 may break, or unite, the images produced by the reflec- 

 tions of the mirrors, there may be represented a whole 

 region : also any part of it may be augmented, so that a 

 small object may be discerned as plainly as if it were close 

 to the observer, though it may be as far distant as the eye 

 can descrie.' In the preface to the second edition (1591) 

 the editor, who was the author's son, affirms that ' by 

 porportional mirrors placed at convenient angles, his 

 lather could discover things far off, that he could knovr 

 a man at. the distance of three miles, and could read the 

 superscriptions on coins deposited in the open fields.' 

 There is probably some exaggeration in this account, but 

 it is sufficiently manifest from it that reflecting telescopes, 

 or optical instruments containing combinations of mirrors 

 and lenses, were known in England before the end of the 

 sixteenth century. The claim of BaptisUi Porta 'between 

 .nd Ilil'i to the invention of the telescope appears 

 to have no other foundation than the circumstance that 



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