436 M. J. Plateau on some new and curious applications 



openings upon thesectorsof this colour continually increasing, 

 the tint of the apparent surface will become more and more 

 vivid, and will pass at last into a brilliant red when the open- 

 ings project entire upon the red sectors. This tint will 

 continue without alteration until the openings begin to en- 

 croach upon the white sectors ; the red will then begin to 

 grow pale, will pass slowly into a rose colour, becoming gra- 

 dually of a lighter shade, and finally to white. This last will 

 then gradually change into a blue, more and more vivid, 

 which after some time will begin to darken, giving place in- 

 sensibly to black. Lastly, if the movement be continued, the 

 phsenomena will be reproduced in the same order. 



The tints thus produced are very beautiful, without having 

 too much brilliancy ; their uniformity is perfect, and the pas* 

 sage from one to the other is effected with extreme delicacy. 

 This illusion then presents, within certain limits, a sort of 

 realization of the ocular harpsichord of Father Castel, and 

 the persons to whom 1 have shown it have appeared to derive 

 from it great pleasure. 



In conclusion 1 would urge this point, that the success of the 

 experiment entirely depends on the establishment of a suitable 

 inequality between the velocities of the two discs. To attain 

 this, it is necessary that, in the construction of the instrument, 

 the diameters of the two brass pulleys should be made as 

 perfectly equal as possible, as well as those of the two wooden 

 pulleys, and that great care should be taken to give the same 

 tension to the two cords, which should be taken from the same 

 piece; the small differences which will always exist, in spite 

 of these precautions, will produce, unless by a very peculiar 

 chance, a sufficient inequality between the two velocities ; for, 

 it will be understood, it is necessary that this inequality should 

 be the least possible. 



LVII. Second Note upon some new and curious applications 

 of the Permanence of Impressions on the Retina. By J. 

 Plateau, Member of the Royal Academy of Belgium^, 

 [With a Plate.] 



WHEN, under the name of Anorthoscopeff I described 

 the instrument intended to produce a peculiar kind of 

 anamorphoses by means of two discs rotating rapidly one 

 before the other, the hind one of which is transparent and 

 bears distorted figures, whilst the front one is opake and 

 pierced with a small number of narrow slits, I made no allu- 

 * From the Bulletin de I* Acad, de Bruxelles, vol. xvi. p. 588. 

 f Bulletin de r Academic, vol. iii. p. 7, year 1836. 



