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IX. Explanation of an Optical Illusion. 

 By Sir David Brewster, K.H., F.R.S., % V.P.R.S. Edin.* 



ABOUT eleven years ago I received from a correspondent 

 well-instructed in optics, a letter informing me that he 

 had composed an essay " On the seeming anomalies which take 

 place in the vision of persons whose eyes are either long-sighted 

 or short-sighted, and which, though they are most truly surpri- 

 sing, have hitherto, as far as I can find, escaped observation." 



" The leading fact," he adds, " which gave rise to the com- 

 position of this essay is as follows : — 



" If a silhouette or black profile of the human face (the fea- 

 tures of which should be very little prominent) looking from 

 right to left be placed against a window, and be viewed by a 

 short-sighted eye through a narrow slit (about the thirtieth of 

 an inch wide) in a sheet of black cardboard placed at some di- 

 stance (about a foot or so) from the window, while the spectator 

 himself is at about the same distance from the slit, that silhouette 

 will be seen by that short-sighted eye looking from left to right." 



After an elaborate investigation of the progress of the rays 

 before reaching the retina, founded upon the known structure of 

 the eye, he deduces a fundamental proposition, " from which, by 

 means of a train of demonstrations (which he has not given) he 

 shows — 



"1. If a silhouette is fixed in a window looking from left to 

 right, and a short-sighted spectator stands with one eye shut at 

 some distance (two feet or so) from it, and moves a piece of black 

 cardboard with a smooth edge, at about two-thirds of the way 

 between him and the window, from right to left, then as the 

 left edge of that piece of cardboard approaches the silhouette, a 

 second or phantomic silhouette will appear in that left edge looking 

 from right to left, so that the two silhouettes will look each 

 •other in the face; and this astonishing appearance takes place 

 accordingly." 



My correspondent deduces from his train of demonstrations 

 other two results, in one of which the cardboard and the silhou- 

 ette are made to change places, and are held at greater distances 

 than before. In this case " the phantom silhouette appears to 

 come out behind the real silhouette, and to look in the same direc- 

 tion." In the third case analogous phenomena are seen by long- 

 sighted persons, the distances of the card and the silhouette being 

 varied. 



Not having seen the demonstrations above referred to, I can- 

 not of course stale that they contain an explanation of these 

 apparently very extraordinary phenomena; but upon carefully 

 repeating the experiments, Isoon saw that the phantom silhouettes 

 * Communicated by the Author. 



