102 On the Optical Illusion of the 



perpendicularly, and brought it in such a manner, that its whole 

 surface almost covered the last glass of the tube ; and in 

 like manner it happened under the compound microscope. 

 But as often as I viewed any of the other objects depending 

 perpendicularly from a perpendicular plane, in such a manner 

 that the tube was supported in a horizontal situation directly 

 opposite to it, the same always happened, and the appearance 

 was not altered, when the object hung obliquely or even hori- 

 zontally. I was mightily delighted with the observation of a 

 tobacco-pipe, which had a porcelain bowl of a snowy whiteness, 

 and a tube of horn almost black, and hung obliquely from a 

 beam ; the bowl preserved its natural convexity, and the tube 

 was deeply sunk, and seemed to be almost immersed in the 

 wall. I also observed, that when I placed the watch horizon- 

 tally upon a horizontal plane, and then looked on it perpendi- 

 cularly, near the window, it no longer appeared so depressed, 

 and surrounded with a shady ring ; whence I began to suspect, 

 that all thosejallacies were owing to shade, just as painters can 

 elevate or depress a figure, by making the ground lighter or 

 deeper. Thus, when the raised object was so placed between 

 the windows, that it must be illuminated on all sides, it did not 

 change its convexity. But at last I discovered a method of 

 making objects always appear with their natural convexity. 

 If any object hung against a wall, or was contiguous to it in 

 any situation whatsoever, I viewed sideways, in such a man- 

 ner as not to oppose the tube directly against it, but below 

 the eminence near the plain at some distance. By those means, 

 the protuberance of the instrument and other objects always 

 appeared to me of their true natural convexity. With regard 

 to the seal, I held it in such a manner, that the whole circum- 

 ference was perpendicular, or rather a little inclined. Then I 

 applied the lower side of the tube exactly to the upper margin 

 of the disc of the seal, so that the tube formed an obtuse an- 

 gle with the seal; then, carefully preserving the same situa- 

 tion, I very gently raised the tube from the rim of the seal 

 upon its face ; and then I always saw the seal with its true na- 

 tural face. But why all these things happen exactly after the 

 same manner, I do not pretend to determine ; nor why white, 

 or uncoloured transparent bodies, rising in any manner above 



