m a Zh'WJomg or engraved Figure of a Crystal. 337 



of the eye foi* obtaining distinct vision. And that whenever 

 the point of distinct vision on the retina was directed on the 

 angle A, for instance, this angle seen more distinctly than 

 the others was naturally supposed to be nearer and fore- 

 most; while the other angles seen indistinctly were supposed 

 to be further, and behind. The reverse took place when the 

 point of distinct vision was brought to bear upon the angle X. 

 This solution being found, I proved that it was the real one 

 by three different ways. 



1st, By being able at my will to see the solid in which posi- 

 tion I chose, and to make this position vary at pleasure, in 

 looking alternately, with fixed attention, either to the angle A, 

 or to the angle X. 



Sndly, While looking steadfastly to the angle A, and seeing 

 the rhomboid in its proper position with the angle A fore- 

 most, if without moving either the eye or the figure, I made 

 a convex lens (such as is used in spectacles for long-sighted- 

 ness.) pass gently from below upwards between the eye and 

 the figure, at the instant when the figure was visible through 

 the glass, the change had taken place, and the solid had assu- 

 med the apparent position in which the angle X was the fore- 

 most, and that only because, owing to the refraction through 

 the glass, the image of the angle X had come to take the place 

 of the real angle A, and so the point of distinct vision, with- 

 out being at all moved, had by this means come to bear on 

 the angle X, or rather on its image. 



3rdly, If through a hole made with a pin in a card you look 

 at the figure in such a manner that either the angle A or the 

 angle X be hidden, the visible angle will determine the appa- 

 rent position of the solid, so that this angle will always appear 

 the nearest; it will be impossible to see it in any other way, 

 and consequently there will be no change. 



What 1 have said of the solid angles is equally true of 

 the edges, — those edges upon which the axis of the eye or the 

 central hole of the retina are directed will always appear for- 

 ward ; so that now it appears to me certain that this little, at 

 first so puzzling, phaenomenon, depends upon the law of di- 

 stinct vision. 



You surely will draw from all the above communications, 

 many consequences which my ignorance of the subject pre- 

 vents me from anticipating. You may do what you think 

 most proper with all these observations. 



I remain, my dear Sir, with the kindest regard, 

 Ever most sincerely yours, 



Geneva, May 24, 1832. L. A. Neckeu. 



Third Series. Vol. 1. No. 5. Nov. 1832. 2 X 



