Instrument foT exhibiting a certain Optical deception. 311 



son why the concavity is invariably upward," when the bars are sta- 

 tionary, and the wheel has both a rotary and a progressive motion, 

 is that these two motions have a fixed relation to each other, the 

 whole wheel in its progression moving in the same direction as the 

 top of the wheel in its rotation. 



While I was seeking to devise some simple apparatus by which 

 this phenomenon might be exhibited as an optical experiment, I first 

 met with Prof. Stamphe's amusing invention, called the Phantas- 

 cope, or Kaleidorama. This at once suggested an easy method of 

 representing the appearance just described. Having covered with 

 white paper a circular disk of pasteboard, nine inches in diameter, 

 I cut 30 or 40 radial apertures from points near the circumference 

 as far towards the centre as the strength of the material would allow. 

 These apertures are an eighth of an inch wide ; and on one side of 

 the disk a heavy ink line is drawn fi'om the inner extremity of each 

 aperture to a small black circle at the centre. The disk, thus pre- 

 pared, has the appearance of being marked \\'ith dark radiating lines 

 from the centre to the circumference. It is then fixed on an axis, 

 and used precisely like the kaleidorama. If the observer stand about 

 three feet fi-om a mirror, and hold the disk a foot from his face, so 

 as to look through the highest apertures at the image in the mirror, 

 on revolving the disk, he will perceive the radii remaining fixed in 

 their position, and very much curved. Reversing the direction of 

 revolution occasions no change ; the concavity in either case is down- 

 ward, both on the right and left, while the vertical lines are straight. 

 But on raising the disk, till the eye sees the image through the low- 

 er apertures, the whole is reversed ; and a change of any number of 

 degrees in the position of the eye produces an equal change in the 

 figure, the outer extremities of the radii being in every case bent 

 away from that part of the circumference which is nearest the eye. 

 As the observer retires from the disk, the curvature rapidly increas- 

 es, till at length in certain positions he sees but three or four of the 

 lines, and those extending through 180° of the disk, with both ex- 

 tremities at the centre, and bearing a striking resemblance to a sys- 

 tem of magnetic curves. 



The general explanation given by Dr. Roget will apply to all the 

 varieties of the phenomenon, which this little instrument exhibits, 

 although only the simplest cases were contemplated by him in his 

 communication. 



Amherst College, Nov., 1834. 



