HI 



ASTRONOMY. 



[ACHROMATIC I.EKSKI 



m, of the fociu, o b, after which it will form a pyramid 

 s, tha divergence of which will bo lessened by pass- 

 Fig. ItX 



ing tnrough the eye-piece in such a manner that they 

 will appear to cmno fnun the point m' in the image a' '/. 

 If tin- rye viewed the imago a 6, without the interposition 

 of the eye-piece, it would only receive a portion of the 

 li-,'ht and rays emitted from the point M, many of the 

 diverging rays having born lust, the opening of the 

 eye being too small to take the whole in. But by employ i n ' 

 a second convex glass, the divergence is diminished, and 

 the eye receives the whole of the light emitted from the 

 point M, which falls on the object-glass, Comparing, 

 therefore, the brightness of an object, as seen with tin; 

 naked eye and seen through a telescope, and mpp- 

 in the latter case, that all the light passes through the 

 lenses, it would follow that the light received by th. 

 in using a telescope, would be as many times gr< 

 than that received by the unassisted eye, as the surface 

 of the object-glass exceeds that of the pupil of the 

 This, however, is only theoretically true, for there is a 

 considerable absorption of light etfected by its pa- 

 through the lenses, as well as by reflection from their 

 surfaces. If the size of the objects were increased by the 

 telescope in the same proportion as the brightness of their 

 surf.-u.vs, it would follow that the brightness would al- 

 ways remain the same. The magnifying power, how. 

 is quite distinct from the illuminative power of the tele- 

 scope, the first depending on the proportion between the 

 focal distance of the two lenses, the latter upon the area 

 of the object-glass. It thus happens that the brightness 

 of the image will vary greatly with the power applied ; 

 and, with high powers applied to the same telescope, the 

 objects become dim and indefinite, and the two excel- 

 lences of brightness and good definition are very difficult 

 to be obtained. 



This description of telescope was not, however, the 

 first invented. The optical principles on which its con- 

 struction is founded were not started by Kepler until 

 some time after the discovery of Galileo. In Galileo's 

 arrangement, the object-glass, as in all other instruments 

 of this sort, is a convex glass, but the eye-piece is aeon- 

 cave one, the focus of which is very short. The disposi- 

 tion of the lenses it seen by the diagram (Fig. 100). The 



Fig. 108. 



reversed image of distant objects produced by the object- 

 glass L is formed in a b, but the concave eye-piece is 

 placed between the two, in such a manner thiit the eye 

 placed behind the lens L', will nee a' 6', instead of the 

 image 6 o ; and the eye-piece can be so adjusted that the 

 image of V will appear well denned. This telescoi 

 nifiw the apparent diameter of objects as many t im - ;.- 

 the focal distance of the eye-piece is contained in tin- 

 focal distance of the object-glass. It will not admit of 

 any considerable magnifying powers, the field of view 

 being very small, on which account it is never used at the 

 present time as an astronomical telescope. On the 

 other hand, it is very convenient as a pocket telescope, as 

 it does not reverse objects ; and as the eye-piece is placed 

 between the object-glass and its focus, it is shorter than 

 the common telescope. In the common opera glasses, 

 whii-h are of thin construction, the magnifying power 

 does not exceed three ; but in some of those made by 



C.i'iloo the magnifying power reached to thirty-two. 

 .IT form of the en-ctinx or terrestrial telescope is 

 that imagined by Hheita, in which three cor 

 glasses, of equal but short focus, a.- fixed in a 

 tube, and form the eye-piece. The first of those 

 lenses, or that most distant from the eye, would 

 give a reversed image, which, pasting through the 

 second one, produces an erect image, and finally, 

 as it is before the focus of the third lens, it is 

 viewed and magnified by this one. The magnifying 

 ].wer of this is measured as in the preceding ones, 

 being in the proportion of the focal distances of the object- 

 glasses and one of the lenses of the eye-piece. The mag 

 nifying ]>ower of a telescope may be roughly determined 

 by comparing the sizes of an object, such as the moon, 

 as seen with it and with the naked eye, and the extent of 

 the field of view may be found by taking the diameter < >f 

 the moon at about half a degree.* The former, however, 

 is determined more exactly by an instrument called the 

 dyanomctur, which serves, when a telescope is dir. 

 towards the sky, and a sheet of paper is hold behind the 

 eye-]'i <Msure the diameter of the luminous circle 



which falls upon the white surface, and the best detini- 

 tion of which is found by trial. By dividing the diam- 

 eter of the object-glass by the diameter of this circle, 

 the magnifying power of the telescope is obtained. Thn 

 luminous circle is itself an image of the object-glass, and 

 is contained as many times in the latter as the telescope 

 niagn" eta. 



Hitherto the object-glass has been supposed to In- 

 formed of one piece and of one sort of glass, but in -,< I 

 MI instruments this is not the case. The image 

 formed by a convex glass is situated at a greater or :> 



.nling to the ivfractii. i' the 



glass, and is so much shorter as the refi .; >T is 



more considerable. In addition to this, the different 

 colours of which white light is composed, are not equally 

 refracted, and cannot have the same focus. If the r\ 

 an observer be situated at the focus of the lens, he will 

 not see the image perfectly defined and colourless, but it 

 will appear surrounded with the prismatic colours ; the 

 violet rays of the object are more strongly refracted than 

 the red ones, and will be thrown nearer the lens. In ad- 

 dition to this source of indistinctness of the image, there 

 is another less important one known by the name of 

 iplierical ah: notion, which depends on the figure of the 

 lens, there being no curvature in which all the rays of 

 light coming from any object are exact!;/ united in a 

 common focus. In order to get rid of the first source of 

 indistinctness, in using a single lens for the object-glass, 

 it was found necessary, with high magnifying powers, to 

 have the lens of very long focus (some of those used by 

 the earliest observers were 300 feet in length) ; for by 

 this means the causes which led to the formation of the 

 colour, viz. , the considerable curvature of the lens, were 

 diminished, and the larger images were formed. By an 

 examination of different species of glass, Dollond found 

 that some sorts refracted light, and dispersed the colours, 

 much more than ordinary glass ; and by passing a ray of 

 light through two prisms made of different glass, he had 

 the satisfaction of finding that a white light was trans- 

 mitted. After this, he employed the two sorts of glass in 

 forming an object-glass, which would transmit r\t. 167. 

 light without decomposing it. This he effected 

 by uniting a convex lens of crown-glass, of 

 n greenish colour, with a concave lens of 

 (lint-glass, of a white tint (Fig. 107). When 

 a ray of light falls upon this object-glass, 

 it is acted upon by both that of the 

 crown-glass renders it convergent, and decom- 

 poses it ; the concave Hint lens, on the 

 trary, destroys the effect of the first, and :m 

 uncoloured image is formed at the focus, liy 

 this beautiful discovery, the refracting tele- 

 has been made a convenient and easily 

 managed instrument, to which higher powers can now 



The olwrrer ihonld not try thin oxpcrimi-nt br rtiri-rtin tlic lolf- 

 copt to tbc nun, a, if nhining lully, it light and heat would MMf the 

 pupil of UK eye. EDITOR. 



