Description of the Kaleidophone. 351 



appear in parallel planes, the furthest image appearing through 

 the first apparent surface. Small pictures have a singular effect 

 apphed in this manner. 



When other sounds co-exist with the fundamental, the images 

 are multiplied, but they become fainter as their number increases : 

 these multiplied images are equally visible, whether the vibra- 

 tions be rectilineal, elliptical, or circular. 



A New Optical Experiment. 



As that property of vision which occasions the apparent 

 duration in the same places of visible images, after the objects 

 which excite them have changed their positions,* has enabled us 

 to submit to inspection the phenomena above described, it may 

 not be irrelevant to subjoin a description of an apparatus which 

 illustrates the transient duration of the impressions of light in a 

 very evident manner. 



At the back of a Avooden frame, about six inches in height 

 and breadth, and from one to three inches or more in depth, 

 a circular plate of glass is placed, upon which a design is painted 

 with transparent colours ; at the front, is placed, parallel to the 

 glass, a circle of tin, covered on its exterior surface with white 

 paper, and having the space between two adjacent radii cut out. 

 This circle moves freely on its centre round an axis, supported 

 by a bar in front, and is put into rapid and regular motion by 

 the application of any mechanical principle proper for the pur- 

 pose ; and a catch is so placed, that when the motion ceases, the 

 aperture shall be concealed by the bar which supports the axis. 



If a light be placed behind the transparent painting, and 

 still better if it be concentrated by a lens, on making the circle 

 revolve with rapidity, the whole of the picture will be rendered 

 visible at one view, although but very limited portions are 

 successively presented to the eye. 



The intensity will differ in proportion to the excess of the 



transmitted light above that which falls in front of the circle ; it 



will, therefore, increase the distinctness of the picture, to darken 



the latter as much as possible. 



* It has l^een proved by the Chevalier d'Arcy, from the only experi- 

 ments approaching to accuracy which have been made on this sul^'ect, 

 that the extent of this duration is eight thirds. See his ** Memoire sur la 

 dur^e de la sensation de la vue : " Hist, de TAcad. An. ] 765. 



