1818.]. £>>'■ Roget on the Kaleidoscope. 375 



the Journal de Physique for February, that M. Vauquelin has 

 also procured the new alkali, and has recognized in it peculiar 

 properties, distinct from those of potash and soda. In addition 

 to the information which we have obtained concerning it from 

 other quarters, M. Vauquelin states, that when it is united td 

 sulphur it produces a vellow sulphuret, which is very soluble in 

 water, and that it contains 43*5 of oxygen, a greater proportion 

 than is found in the other alkalies. M. Vauquelin has not been 

 able to detect any litlhon in the albite of Sweden. M. Haiiy 

 has determined the primitive form of the petalite to be a right 

 rhomboidal prism, of which the rhomb is longer than any which 

 have yet been examined. 



The petalite was first described by M. Dandrada, in the 

 Journal de Physique ; * but the account given of it was so im- 

 perfect that it has been little attended to, until M. Swediensterna 

 lately sent some specimens of it both to France and to this 

 country. 



Article XV. 

 On the Kaleidoscope. By P. M. Roget, M.D. F.H.S. &c. 



(To the Editors of the Annals of Philosophy.) 



GENTLEMEN, Bei-nard-street, Russel-square, April 3, 1818. 



The amusing optical instrument for which Dr. Brewster has 

 lately procured a patent, and to which he has given the veiy 

 appropriate name of kaleidoscope, admits, as stated in the spe- 

 cification, of various forms of construction. Those composed of 

 more than two mirrors, and which may be. denominated polygonal 

 kaleidoscopes, have not, however, been so particularly noticed by 

 the inventor, as their superior practical utility when applied to 

 the arts would seem to deserve. Some inquiry into the princi- 

 ples on which they should be constructed may, therefore, be not 

 unworthy of occupying a place in your journal. 



The principle of the instrument in its common or simplest 

 form, is the formation of a series of images disposed in the cir- 

 cumference of a circle by the multiplied reflections of a set of 

 objects from two plane mirrors inclined at a certain anc;le. It is 

 evident that in order to obtain a regular appearance throughout 

 the field of view, this angle must be an equal division of the 

 circumference, otherwise a broken portion of a sector of the 

 eircle will present itself at the opposite part of the field from 

 that which the objects themselves occupy. It is not quite so 

 obvious, however, that this angle must not merely be an aliquot 

 part of the circumference, but that it must also divide the semi- 

 circle equally. If this latter condition be not observed, portions 



♦ Vol. li. p. 244. 



