MOTION OF THE FEATUEES OF A POKTEAIT. 123 



look at the spectator. Let a straight line be 

 drawn through the tip of the nose and half way 

 between the eyes, which we shall call the middle 

 line, On each side of this middle line there will 

 be the same breadth of head, of cheek, of chin, 

 and of neck, and each iris will be in the middle 

 of the white of the eye. If we now go to one 

 side, the apparent horizontal breadth of every 

 part of the head and face will be diminished, but 

 the parts on each side of the middle line will be 

 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 white of the eyeball, so that 

 the portrait preserves all the characters of a 

 figure looking at the spectator, and must neces- 

 sarily do so wherever he stands. 



This explanation might be illustrated by a pic- 

 ture which represents three artillerymen, each 

 firing a piece of ordnance in parallel directions. 

 Let the gun of the middle one be pointed accu- 

 rately to the eye of the spectator, 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 spec- 

 tator 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 



