MOTION OF THE FEATURES OF A PORTRAIT. 195 



diminished equally, and at any position, however oblique, 

 there will be the same breadth of face on each side of the 

 middle line, and the iris will be in the centre of the 

 whole of the eyeball, so that the portrait preserves all 

 the characters of a figure looking at the spectator, and 

 must necessarily do so wherever he stands. 



This explanation might be illustrated by a picture 

 which represents three artillerymen, each firing a piece 

 of ordnance in parallel directions. Let the gun of the 

 middle one be pointed accurately to the eye of the specta- 

 tor, so that he sees neither its right side nor its left, nor 

 its upper nor its under side, but directly down its muzzle, 

 so that if there was an opening in the breech he would 

 see through it. In like manner the spectator will see the 

 left side of the gun on his left hand, and the right side of 

 the gun on his right hand. If the spectator now changes 

 his place, and takes ever such an oblique position, either 

 laterally or vertically, he must still see the same thing, 

 because nothing else is presented to his view. The gun 

 of the middle soldier must always point to his eye, and 

 the other guns to the right and left of him. They must 

 therefore all three seem to move as he moves, and follow 

 his eye in all its changes of place. The same observa- 

 tions 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 canvas being 

 imperfectly stretched, as the slightest concavity or con- 

 vexity entirely deforms the face when the obliquity is 

 considerable. The deception 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 obliquities. This 

 distinctness of outline is indeed necessary to a satisfactory 

 exhibition of this optical illusion. The most perfect 



