BY WAY OF INTRODUCTION 3 



of the circular " window " immediately above 

 the observer. 



Let us first consider the white gull flying 

 overhead, the large round boulder on the bottom, 

 to the right, and the submerged water-lily leaves 

 beyond. 



From the point c the gull appears as shown 

 on the lower illustration ; that is, as a dark sil- 

 houette flying across the " window," with the 

 fleecy clouds beyond. The gull swimming on 

 the surface and the penguin and fish under the 

 .water will be referred to later. Outside the 

 circle the surface of the water reflects the dark 

 bed of the loch, but the reflected images of the 

 round boulder and water-lily leaves are alone 

 illustrated. Now look again at the top illustra- 

 tion. A gull is shown swimming on the surface 

 beyond the circle ; a sailing boat is seen coming 

 down the loch, and at the water's edge a fisher- 

 man awaits the arrival of the boat. Seen from 

 below the water, that portion of the gull beneath 

 the surface is seen as a dark object on the left, 

 marked by a white cross, while the image of the 

 head appears on the edge of the circular 

 "window." The hull of the boat cannot be 

 seen under the water, for it is lost in the dark 



