124 LETTERS ON NATURAL MAGIC. 



seem to move as he moves, and follow his eye in 

 all its changes of place. The same observations 

 are of course applicable to buildings and streets 

 seen in perspective. 



In common portraits the apparent motion of the 

 head is generally rendered indistinct by the can- 

 vas being imperfctly stretched, as the slightest 

 concavity or convexity entirely deforms the face 

 when the obliquity is considerable. The de- 

 ception is therefore best seen when the painting 

 is executed on a very flat board, and in colours 

 sufficiently vivid to represent every line in the 

 face with tolerable distinctness at great obli- 

 quities. This distinctness of outline is indeed 

 necessary to a satisfactory exhibition of this 

 optical illusion. The most perfect exhibition, 

 indeed, that lever saw of it was in the case of a 

 painting of a ship upon a sign-board executed in 

 strongly gilt lines. It contained a view of the 

 stern and side of a ship in the stocks, and, owing 

 to the flatness of the board and the brightness of 

 the lines, the gradual development of the figure, 

 from the most violent foreshortening at great ob- 

 liquities till it attained its perfect form, was an 

 effect which surprised every person that saw it. 



The only other optical illusion which our limits 

 will permit us to explain, is the very remarkable 

 experiment of what may be truly called breathing 

 light or darkness. Let S be a candle whose light 

 falls at an angle of 56 45' upon two glass plates 

 A, B, placed close to each other, and let the re- 

 flected rays A C, B D, fall at the same angle upon 

 two similar plates, C, D, but so placed that the 

 plane of reflexion from the latter is at right angles 

 to the plane of reflexion from the former. An 



