72 



UNDULATORY FORCES. LIGHT. 



rrKH>ro!->:s. 



of form, .to., which arise from the different purposes to 

 which it is applied, I'.efore entering ou this subject, tlio 

 student should li:ivo become familiar with the priii 

 un which lenses and curved reflectors are constructed, 

 and which we fully explained in the earlier pages of this 

 work.* 



Telescopes are of two kinds ; namely, refracting and 

 reflecting. In the former, the rays of light passing from 

 the object are successively refracted by two or more 

 lenses before reaching the eye. Of these, again, there 

 are two classes, the ordinary and achromatic ; in the 

 latter, the coloured fringes produced by the employment 

 of lenses of one kind of glass, as in common telescopes, 

 are avoided by the use of lenses composed of two kinds ; 

 namely, crown and flint, which, by their different dis- 

 persive powers, destroy their natural tendency to produce 

 coloured images. The principles on which achromatic 

 lenses are constructed have been fully explained in our 

 previous pages ;t to which, for the sake of saving space, 

 we must refer our readers. Reflecting telescopes, on the 

 other hand, receive the rays passing from a distant object 

 on a peculiarly curved reflector, called a speculum ; and 

 these are reflected to the eye through an eye-piece. 

 Hence, generally speaking, reflecting telescopes combine 

 the principles of both reflection and refraction in their 

 construction. Their forms have been varied according to 

 the special views or requirements of their inventors. 

 We shall select those most known and appreciated as 

 the subject of our explanation ; and give a general outline 

 of the means employed to cast and to grind the speculum 

 to a proper shape. 



REFRACTING TELESCOPES. 



FROM the time of Galileo, to whom is generally accorded 

 the merit of their discovery, telescopes of this kind have 

 been the object of great care on the part of the instrument- 

 inakera. The value of a telescope depends, essentially, 

 mi its affording a well-defined, colourless, and correct 

 imago of the object viewed by it ; and these results are 

 only obtained by the employment of lenses as free as 

 possible from either spherical or chromatic aberration. J 



For ordinary purposes a combination of lenses is re- 

 quired, which will present the image, to the eye of an 

 observer, in a vertical position ; or, in other words, the 

 oliject must appear, in all respects, with the exception of 

 size, as if it were viewed by the naked eye. In instru- 

 ments used for astronomical purposes, this precaution or 

 peculiar arrangement is unnecessary, because size and 

 position are the cliief objects which are sought after; 

 hence, two lenses are sufficient in many cases. 



Before proceeding to describe various forms of the 

 refracting telescope, it may be as well that we should 

 call the attention of the reader to the names of the two 

 I'hief lenses the eye-piece and the object-glass. The 

 former term is applied to the lens which is placed next to 

 the eye of the observer ; whilst the object-ylass is that lens 

 furthest from the eye, and next to the object; it, indeed, 

 first receives the rays of light entering the instrument. 

 The definition of these terms will save us much circum- 

 locution hereafter. 



As nothing conduces more to interest the student in 

 physical science than assisting him to the personal appli- 

 cation of its laws and principles, we shall now give 

 directions, by means of which any one may construct a 

 simple telescope at a trifling expense, and yet of sufficient 

 power to enable liim to view many interesting astro- 

 nomical appearances. We are indebted to Dr. Dick for 

 the following description of an instrument of this kind, 

 which we have slightly modified. 

 Fig. so. 



.0 



"Procure a double convex lens of about thirty-six 

 inches focal length, and place it at the end of a tube, as 



See ante, p. <3, it ley. -f Ante, p. 53. J Ibid. 



at A B, Fig. 50. At thirty-seven inches distant from 

 this lens fix another, as at C D, whose focal length is one 

 inch. This should bo fixed in a separate short tube, 

 E F, which should be so made as to slide backwards and 

 forwards in the large tube, for the purpose of adjusting 

 the focus of the telescope to the eye. The folio 

 particulars require to be attended to. The aperture or 

 opening which lets in the light at A B (that is, in front of 

 the lens), should not exceed one inch in diameter, other- 

 wise the image of the object will be somewhat confused. 

 There should be a hole (or stop) in the focus of tin- 

 glass, C D, rather less in diameter than the breadth of 

 the lens, for the purpose of excluding extraneous rays." 



The tubes may be made of tin or zinc, and the insides 

 should bo blackened by painting them with a mixture of 

 lamp-black and size. The stops at each end of the 

 are easily made out of circular pieces of zinc, from the 

 centre of each of wliich a neat round hole must be cut ; 

 that for the object-glass being an inch in diameter ; and 

 that for the eye-glass may be about half au inrh. 

 These can easily be soldered into their plaees on the 

 tube. Such an instrument can be made for five slii 

 and it will have a magnifying power of thirty-six times. 

 Of the question of magnifying power, however, we shall 

 speak more fully as we proceed. Simple as this form of 

 telescope is, we may inform our readers that, in using it, 

 they will be enabled to observe Saturn's ring, Jupiter's 

 satellites and his belts, the lunar mountains, and the 

 spots on the surface of the sun. For the latter purpose, 

 however, the eye should be protected by means of a 

 piece of dark-blue or blackened glass, placed between the 

 eye and the eye-piece of the telescope, or the intensity 

 of light and heat would be seriously injurious. \\ o 

 must also impress on all who may attempt to make 

 such an instrument, the necessity of having the axes of 

 the two lenses in exactly the same line. In other words, 

 a line drawn tlirough the centre of the object-glass A B, 

 must, in a properly-constructed instrument, also pass 

 through the centre of the eye-piece, C D. In the con- 

 struction of optical instruments of all kinds, wherein two 

 or more lenses are employed, this is certainly the " pons 

 rum;" and neglect of this simple rule oiteii 

 involves a vast amount of annoyance and expense. Of 

 this we met with an amusing instance in the case of a 

 friend, who had expended upwards of thirty pounds in 

 remedying a vast number of imaginary defects in an 

 oxy-hydrogen microscope. On inspection, we found that 

 the only fault existing was, that a brass arm, at each 

 end of which a lens was fixed, required to be straightened, 

 an operation which was performed in a few seconds by 

 the aid of a vice. , 



We must now explain the cause and extent of the 

 magnifying power of telescopes; and the simple one 

 which we have just described will enable us to do so 

 without danger of distracting the attention of the student 

 by unnecessary detail, because the principles involved in 

 its construction are equally so in every ot'ier form of 

 telescope. In this telescope, the eye-piece is the medium 

 by which the image produced by the object-glass is 

 viewed and magnified ; hence the magnifying power of 

 any telescope of this kind will be in the ratio which 

 exists between the focal distances of these two lei 

 For instance, the focal length of the eye-piece, in the 

 instrument we have described, is supposed to be one 

 inch, whilst that of the object-glass is thirty-six inches ; 

 we thus obtain a magnifying power of thirty-six times. 

 It also appears, that if the focal length of the object- 

 glass is increased, and that of the eye-piece diminished, 

 the magnifying power of the telescope is increased in the 

 ratio which these two measurements bear to each other. 

 We should here observe, that when we speak of the 

 power of a telescope as magnifying so many times, it is 

 meant that the diameter, or linear mea-uiv.n. nt, is 

 increased. Of course, the total superficial result will 

 equal the square of the linear one. Our telescope, for 

 instance, is supposed to have a linear magnifying power 

 of thirty-six times; but its superficial power will lie 

 equal to the square of 30, or 1,290 tr. 



In the following diagram, we have an illustration of the 



