THE EYE AND THE PHYSIOLOGY OF VISION. 589 



was miles away it seemed to reach into the clouds. A 

 second reason for the increased size of the moon lies in the 

 comparison which we make of the size of the same with the 

 objects in its range. We see for instance the moon about 

 twice as wide as a certain chimney, the dimensions of which 

 we know, and in an unconscious way we make calculations 

 as to the moon's size. When the moon is overhead there 

 are no intervening objects and the comparison does not 

 occur. 



5. Shine or Brilliancy. It was pointed out in the dis- 

 cussion of the reflection of light that a perfectly smooth 

 surface reflected the light in an unbroken way and would 

 appear even to a single eye polished or shiny. But the 

 sensation of the shine or brilliancy of a surface is very 

 much increased when we look at it with both eyes. This 

 seems to be due to the fact that the same amount of light 

 is not reflected into both eyes. It is apparent that the 

 reflection from a shining surface is not the same for differ- 

 ent directions. Especially is this true if that surface should 

 move slightly as, for instance, a surface of water. This 

 slight disagreement between the sensations of brightness 

 in the two eyes is interpreted by the mind as increased 

 shine or brilliancy. It is possible experimentally to take 

 two surfaces, neither of which when seen alone appearing 

 polished, and, placing one before each eye and then illu- 

 minating them with different intensities, to produce a sen- 

 sation of shine or brilliancy. The mind seems to combine 

 the non-agreement of intensity of light reported by the two 

 eyes into a new sensation. In this way arises, for instance, 

 the brilliancy of reflected moonbeams from a rippling sur- 

 face of water. 



6. Entopic Illusions. These are illusions which arise 

 from a cause lying within the eye itself. They are: 



(a) Musccz volitantes. These are commonly called the 

 flying motes, and appear as little black spots which we pro- 

 ject into space. These black spots are due to shadows which 



