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LVI. Oti a new and curious Application of the Permanence 

 of Impressions on the Retina. By M. J. Plateau, Member 

 of the Royal Academy of Belgium*. 



[With a Plate.] 



TWO discs of white paper, of sufficient strength, are to be 

 cut out, one 30 and the other 35 centimetres in diameter. 

 The first is divided into eight equal sectors ; two opposite sec- 

 tors are then painted red, and two others blue, employing for 

 this purpose fine gum colours; two other opposed sectors are 

 afterwards covered with a very opake black, and a circular 

 space of 4 centimetres in diameter, in the centre of the disc, 

 is covered with the same black ; finally, the two last sectors 

 are left white (see Plate IT. fig. 1, in which the colours red and 

 blue are indicated by the shading and the letters r and b). 

 When this is done, a colourless varnish is laid upon the disc, 

 which penetrates into the pores of the paper, and gives more 

 transparency to the white and coloured partsf . In the other 

 disc are made two opposite openings, likewise in the form of 

 sectors, but which extend to only 3 centimetres of the circum- 

 ference of the disc, and whose angular width is only three- 

 fourths of that of the sectors of the other disc ; their extremi- 

 ties adjoining the centre also leave a distance between them of 

 4 centimetres ; this disc is blackened all over (see fig. 2). 



These discs are fixed respectively at their centres, by means 

 of nuts, upon two small brass pulleys 3 centimetres in dia- 

 meter. These last are placed upon a support so that the 

 discs are vertical, parallel to one another, and distant 3 cen- 

 timetres from each other, and so that the two axes are in 

 the same straight line. These pulleys are furnished with 

 cords which pass over two other larger pulleys ; these are of 

 wood ; they are both 15 centimetres in diameter, and are fixed 

 upon a common axis, which is furnished with a small handle. 

 Lastly, the cords are arranged so that the two brass pul- 

 leys, and consequently the two discs, turn in the same direc- 

 tion. This system is similar to that of the anorthoscope^ 

 as it has been presented to the public, except the equality 

 in diameter of the brass pulleys, the community of direction 

 of their motions, the less number of the openings of the 

 black disc, and the greater width of these openings. Like 

 the anorthoscope also, the present instrument must be em- 

 ployed in the evening, and strongly illumined by a good lamp 



• From the Bulletin de I'Acad. de Bruxelles, vol. xvi. p. 424. 



■\- In order that, in the disc thus varnished, the tints of the coloured 

 parts may remain very bright when observed by transmission, it is well for 

 the colours to be applied on both sides of the paper. 



