ERRORS AS TO THE SIZE OF THE MOON 79 



(which stands for us as the equivalent of an artist's picture) 

 the moon will occupy almost exactly one inch in diameter 

 the size of a halfpenny. With such a simple apparatus 

 of peep-hole and smeared glass in an upright frame, it is 

 easy to mark off the size covered by the moon (or sun), 

 whether low or high, on the smeared glass, and it is found 

 never to vary whether high or low so long as the same 

 " eye-to-frame " or " peep-hole " distance is preserved. 

 That seems to be an important fact for painters of sun- 

 sets and moon-rises. But what do they do ? They never 

 give the right size (namely one sixth of an inch) which 

 corresponds to an eye-to-frame distance of eighteen inches. 

 They give to a high moon, if they are very careful, a 

 quarter of an inch for diameter. This means that the 

 observer is about two and a half feet, or thirty inches from 

 the picture nearly twice what the artist's eye really is as 

 he paints. And then-^-if painting a moon-rise or sunset 

 they suddenly pretend to go to a distance of nine and a 

 half feet from the picture and make the moon an inch 

 across because it is low down, or even give the moon two 

 inches in diameter, which would mean that they (and those 

 who look at the picture when hung up for view) are 

 observing at nineteen feet distance from the front plane or 

 frame of the picture. They do not alter the other features 

 in the picture to suit this change of distance of the eye 

 from the frame and there is no warning given. Certainly 

 there is no obvious and necessary reason for treating a 

 picture containing a high moon as though you were three 

 feet from the front plane of the scene presented, and a low 

 moon as though you were twenty feet from that plane ! 

 The confusion which may result in the representation of 

 other objects when these changes of eye-to-frame distance 

 are made is shown by the following simple facts. According 

 to the simple laws of perspective, if the eye is at thirty 

 inches from the picture-plane or frame (as declared by a 



