KALEIDOSCOPE. 



413 



FIAT* 

 CC XL I. 



The principle which we hve now explained, is of 

 primary importance in the construction of the Kalei- 

 doscope, and it is only by a careful attention to it that 

 the instrument can be constructed so as to give to an 

 experienced and fastidious eye that high delight which 

 it never faili to derive from the exhibition of forms per- 

 fectly symmetrical. 



From thee observations it follows, that a picture pos- 

 sessed of mathematical symmetry, cannot lie produced 

 unless the object is placed exactly at the extremity of the 

 reflectors, and that even when this condition is complied 

 with, the object itself must consist of lines all lying in 

 the same plane, and in contact with the reflectors 

 1 it-nee it is obviou-., that objects whose thick nest is 

 perceptible cannoi give mathematically symmetrical 

 patterns, for one side of them must always be at a cer- 

 tain distance from O. The deviation in this case is. 

 however, so small, that it can scarcely be perceived in 

 Objects of moderate thickness. 



In the simple form of the kaleidoscope, the produc- 

 tion of symmetrical pattern* is limited to objects which 

 can be placed rle t.. the aperture AOB ; but it will 

 be seen in the sequel of this article, that this limitation 

 nay be removed by an optical contrivance, which ex- 

 tends indefinitely the use and application of the instru- 

 ment. 



Scr. IV. On the T./tct vrnltieed by rarying the Length 

 of the Rejttctort. 



Before we proceed to investigate the effects produced 

 by a variation in the length of the reflecting plane*, it 

 will be nece*ary to cooMilrr the variation of the inten- 

 sity of the li^tit'iji different nan-, of the reflected tec- 

 tor*. In the direct -*ctor A< >IJ, ( KU 1) the intensi- 

 ty .,i r'urm in every pan of its surface ; 



. am being the CMC in the image* form- 

 1 take any two point* *, o, 



and <lraw the lines m n, o n, perpendicular to AB, and 

 mr. /. 4iid r- !' <)l '-. '' +, be -ection 



of th. cen edgewine and let Op, O , be 



taken equal to the lint-* ma, o />. or the he-ght of the 

 points , p, above t! the reflector AO. Make 



e. the ronttant height of the 



ey above the reflecting plane, an I O r to OK as O * 



r, and the points R.r, will be the |>oinu of iiici- 



from f> aiul n ; for in lliia case 



Hence it \ obvious, 



that IK- i lew than V.rr. ami that the rays iwuing 

 from />, by falling more obliquely upon the refit-rung 

 surface, will be more copiously reflected. It follows, 

 therefore, that the intensity of the light in the reflected 

 sector AO6 is i ', the linen of equal (tightness, 



or the uoakatal lines as they may be called, being parallel 

 to t'le reflecting surface AO, and in every sector parallel 

 to the radius between the given sector and the n fleeting 

 surface by which the sector '* formed. The intensity 

 of the light in diffen nt |x>ii>ts of the smnie sector, aa 

 well a. in the difleri nt sector-, ia affected by the poUri- 

 Mtinn which the light experiences by successive reflrc- 

 tions ; an effect which is produced, though in a difi.-r. nt 

 m .nner, when the reflectors are meUllic. In plates of 



flats, the pencil in the last reflected image a'" O //", 

 i. is polarised in plane perpendicular to MS, or 

 in the same manner as if it had been reflected at the 

 polarising a.igle from a vertical plane p rid lei to M V 



Let us now consider what will take place by a varia- 

 tion in the length of the reflecting planes, the angular 



extent of tl>e field of view remaining always the same. If 

 AOE, AOE', Fig 5 be two reflecting plates of the s-.me 

 breadtn AO. but of different lengths, it is manifest that 

 the light which forms the direct sector must be incident 

 nearer the perpendicular, or reflected at less obliquities Kig. 5. 

 in the short plate than in the long one, and therefore 

 that a similarly situated point in the circular field of the 

 sliorter instrument, would have less intensity of li^lit 

 than a similarly situated point in the larger instrument. 

 But in this case, the field of view in the short instru- 

 ment is proportionally enlarged, so that the comparison 

 between the two 13 incorrect. When the long and the 

 short instrument have equal apertures, which will be 

 the case when the plates are AOE, A'OE, then similar- 

 ly situated points of the two fields will have exactly 

 the same intensity of light. 



There is, therefore, no peculiar advantage derived in 

 thcor\ , from increasing the length of the reflectors be- 

 \ ond the distances at which their extremities can be dis- 

 tinctly seen by the naked eye In practice, how ever, the 

 d.fliculty of constructing a perfect instrument increases 

 with the length of the refl.ctors. The additional risks 

 of dreak ng and bending, and the additional difficulty- 

 t ruling a good junction, render it advisable to li- 

 mit thr greatest length of the planes to six, seven, or 

 eight inches. 



SECT. V. On the Construction and Use of the timplr 

 Kaleidoscope. 



When two reflecting planes are placed in a tube, ac- 

 cording to the principles explained .in the preceding 

 section, and the eye applied to one end of it, it will 

 perceive at the other end a circular field composed of as 

 many luminous sectors as the number of times that the 

 inclination of the plates is contained in 360*. 



If we now fix upon the edge* AO. BO, Fig. 1, some 

 piece I of differently coloured glass, so _-. to project with- 

 in the aperture AOR, the portion, of the glass included 

 in the aperture will also be included in all the im.-iges 

 of the apertures ; the images of these portions in the 

 inverted sectors, will j >ni those in the direct sectors, 

 and the whole will be arranged into a fixed sym- 

 metrical pattern, far surpassing > hat ran IK- produ- 

 ced by the hands of the most ski) till .irti-t. If Wt* now 

 turn thr tube to different |wrti of the room, the 

 light will full indifferent directions ii|>on thefr.-ignirpts 

 of coloured glass, and a variation of the patterns will 

 thus be produced to very considerable degree fr.nn 

 fixed object*. This was the first state in which Dr. 

 Brewtter constructed the kaleidoscope ; and it was not 

 for some time afterward* that tin* idea of varying the 

 pattern by the motion of the objects occurred to him. 



In executing !:e kaleidoscope in this new form, Dr. 

 Brewnter fir-i pi rrl the coloured glass, or other objects, 

 between two pi di- ol the thinnest glass, and holding 

 tfn- nliject plate in th- h.ind, it was moved in various di. 

 rections arrosi the .m. uinr aperture of the instrument. 

 The instrument was still farther improved, by sliding 

 the object plate in a groove, as in the migic lantern, 

 by placing h o'jf-u in a cell at the end of the reflec- 

 tors; or by givint; the object plate a circular form, .>o 

 M to have a rotatory moiion round the axis of the tube. 

 By these means, the power of the kaleidoscope, and the 

 beauty of the pattern, were inconceivably increased ; 

 and from being a mere toy. it now promised to be an 

 useful and .r-.o-.ing instrument 



The kaleidoscope in this simple form, is shewn i* 



