.t 445 ] 



LXXIX. History o/" Dr. Brewster's Kaleidoscope, uilh Re- 

 marks on its supposed Resemblance to other Combinations of 

 plain JSlirrors. 



J.N our last Number we laid before our readers a few observa- 

 tions on this instrument, the result of which was, that the ka- 

 leidoscope is in truth a new invention, unknown to those from 

 Uiiom some have affected to consider Dr. Brewster as having 

 borrowed it. As the instrument excites universal attention, a 

 brief sketch of the history of the invention, for which we are 

 indebted to a friend at Edinburgh, cannot but prove acceptable 

 to our readers. 



In the year 1814, when Dr. Brewster was engaged in experi- 

 ments on the polarization of light by successive reflections Ije- 

 tween plates of glass, which were published in the Philosophical 

 Transactions for 1815, and honoured by the Royal Society of 

 London with the Copley Medal, the reflectors were in some 

 cases inclined to each other, and he had occasion to remark the 

 circular arrangement of the images of a candle round a centre, 

 or the multiplication of the sectors formed by the extremities of 

 the glass plates. In repeating, at a subsequent period, the ex- 

 periments of M. Biot on the action of fluids upon light. Dr. B. 

 placed the fluids in a trough formed by two plates of glass ce- 

 mented together at an angle. The eye being necessarily placed 

 at one end, some of the cement which had been pressed through 

 between the plates appeared to be arranged into a regular fi- 

 gure. Tli« symmetry of this figure being very remarkable. 

 Dr. B. set himself to investigate the cause of the phaenomenon, 

 and in doing this he discovered the leading principles of the 

 kaleidoscope. He found that, in order to produce perfectly 

 beautiful and symmetrical forms, three conditions were necet- 

 sary. 



1 . That the reflectors should be placed at an angle, which was 

 an even or an odd aliquot part of a circle, when the object was 

 regular ; or the even aliquot part of a circle when the object was 

 irregular. 



2. That out of an infinite number of positions for the object 

 both within and without the reflectors, tliere was onhj one posi- 

 tion where perfect synunctry could be obtained, namely, by 

 placing the object in contact with the ends of the reflectors. 



■ u. That out of an infinite number of positions of the eve, 

 Uiere was only 07ie where the symmetry was perfect, nainelv us 

 near as possible to the angular point, so that the circular field 

 could be distinctly seen ; and that this point wa-; the only one 

 out of an infinite number at which the unifoiniity of the light 

 «f the circular field was a maximum. 



I [ion 



