UNDULATOKY KolU'KS. UCIl !'. 



[MIRRORS, SPECTACLES, ETC. 



mi v. in...!.- is that be. 

 .<. It has n i! 



. and its focal length is about 



>sod of copper 



and tin united, in wh:it, in cVmieal language, would 



be termed nto'nio proportions; or, in other words, 



such quantities of I'.ioli metal wore employed as are 



represented by those in which thoy would combine 



with ii< i forming definite compounds. 



Recurring to the proper shape of the Rpociilnm, we 



refer to our remarks on spherical aberration,* and 



also to those on the properties of reflectors which have 



< of one of the conic sections. In obedience to the 



n from curved surfaces, the specula used 



in iv 'scopes are usually nride in the shape of 



n p:ir.i!> '!:i: and thus, as far as possible, the evils of 



d aberration are avoided. 



It will, perhaps, be more convenient that we should 

 now illustrate the principal forms of reflecting telescopes 

 which have been employed; and include, at the same 

 time, the description of the small reflector and eye- 



. specially adapted to their construction. 

 In the annexed figure we have a representation of Sir 

 Isaac Newton's reflecting telescope. In this (see Fig. 

 69), M is the speculum on which the rays from a distant 

 It, impinge. These are reflected from M to N, 

 which is a plane reflector, placed at an angle of 45 

 to the axis of the mirror. The image reflected from 

 N is received and magnified by an eye-glass placed at O. 

 The eye of the observer is of course placed at the 

 opening beneath O, in a position which is extremely in- 

 convenient. The mrrror is placed between the focus a b 

 and the speculum, and the imago is formed at a b'. 



Fig. 59. 



"" 





The Gregorian telescope so called after its inventor, 

 Dr. Gregory has a very different and much more conve- 

 nient arrangement. In using an instrument of tUis kind, 

 the observer views the image in a line which forms the 

 axis, instead of being at right angles thereto, as in Sir 

 Isaac Newton's telescope. A hole is bored in the large 

 speculum, at its centre ; and the rays of light reflected 

 from the surface are received by a smaller concave mir- 

 ror, which again reflects them to the eye-piece, and so 

 they reach the eye of the spectator. Fig. CO illustrates 

 the construction of a Gregorian telescope ; and the same 

 letters are employed as in our last engraving, which will, 

 therefore, at once point out the different parts, their 

 position only being varied. 



Fig. 60. 



The Cassegranian telescope has a convex mirror, placed 

 at N, instead of a concave one, as employed in the ( ire- 

 gorian. I forms of instrument have been vari- 



ously modified, by astronomers and others, since the 

 time of their invention. 



The celebrated sir William Herschel introduced an- 

 other form of reflecting telescope, in which the observer 

 looked down the tu ML; the speculum, and of 



course with hi- M M - ol.jrct. The speculum was 



placed at an angle with the axis of the instrument, and 

 so cast the reflected rays towards one edge of the tube, 

 at which they were viewed by means of an 

 By this arrangement a second reflector, as employed in 

 the ; -Hied instruments, was rendered un- 



necessary. Fig. 01 represents a section of this form of 

 telescope, in which the r.-, g>rocccding from A B are 

 Sec ant", p. 44. 



| received by the angularly placed speculum, an 

 I to a 6, at which uoint they are viewed. 

 Fig. 61. 



A 

 "B 



I 



MISCELLANEOUS OPTICAL INSTRUMENTS. 



HAVIJTO mentioned the principal optical instruments, 

 their construction and their uses, we now proceed to 

 speak of those which, whilst highly interesting and use- 

 ful, are still of comparatively minor importance, and, as 

 such, will not require so extended notice. 



Amongst these wo may include the following : namely, 

 Concave and Convex Mirrors, Spectacles, Magnifying 

 Glasses, Burning Glasses, the Kaleidoscope, Debus.-: 

 Tliaumatrope, Kalotrope, <tc. ; of each of which we shall 

 give a short explanation. 



We have already explained the principles of concave 

 and convex mirrors, and have, to some extent, alluded 

 to their various uses. In doing so, however, we confined 

 ourselves to strictly scientific illustration, and now pro- 

 pose to show a variety of amusing uses to which many 

 of them can be applied. The same may be said of the 

 other instruments to which we have alluded ; all of 

 which are constructed on the principles already laid 

 down modified, however, by peculiarity of construction 

 and object. 



As a variety of interesting experiments may be ' 

 with both concave and convex mirrors, we may hero 

 enter more fully into the practical part of t!i>-' 

 Btructiou, than we have in our previous pages. - Wo 

 A there stated, that unless a parabolic or elliptic form 

 B be employed, great loss of light and heat is snsi 



during reflection. For the purpose, however, of the 



following experiments, those of a sphenr.il shape 



will answer very well. We may add, that such will 



be equally useful to the student during his studies ,,{ 



heat and sound ; because they, like light, obey the laws 



of radiant forces. 



Where the means of the student will permit, we advise 

 the purchase of reflectors having a polished silver surface, 

 which may be obtained of most of the optical instrument- 

 makers. As, however, some of our readers may not be 

 in a position so to do, and it being our desire to facilitate 

 the repetition of all our experiments, the following direc- 

 tions may be useful ; and if carefully carried out, they 

 will enable almost any person to become practically ac- 

 quainted with the laws and amusing applications of 

 curved reflectors. 



Either metal or glass may be used for the purpose. 

 Generally speaking, metallic reflectors are preferred. A 

 sheet of copper is to be hammered into a hollow 

 A shape, which can be readily done by any copper or 

 l tin smith. Instead of copper, tin or zinc may be 

 8 used, as either of these metals will receive a good 



polish. They are, however, liable to tarnish ; and 



just in proportion as that takes place, the value of the 

 reflector for experimental purposes is diminished. The 

 size of the reflectors may vary from eight to twelve inches 

 in diameter, the reflecting power increasing in the ratio 

 of the diameter. If copper reflectors be preferred, thoy 

 should have their surface polished as brightly as possible, 

 and be afterwards silvered by means of the electrotype 

 process, or in the ordinary manner of plating. They 

 then require repolishing; and such have the advantage 

 of retaining the polish for a long time, and also reflect a 

 larger amount of incident rays of light than any other 



ni.-t.il. 



If glass be used, th- urface may be " silvered" 



in a similar manner as is adopted for ordinary looking- 

 glass if a convex mirror is required, and the inside for a 

 concave: in either case, the side opposite to the reflecting 



t Sftanlf, p. 4S, M 



