THE PUNCTUM CECUM. 133 



suddenly turn our eyes towards the banks, it will appear as though the river 

 were flowing back towards its source. This kind of compensation does not 

 only produce an apparent displacement, but also changes in size. When 

 travelling at great speed in a railway train, the objects of the surrounding 

 country as one flies by them gradually appear smaller and smaller. If, when 

 this occurs, we suddenly remove our eyes to the interior of the railway 



Fig. 138. Experiment on complementary colours. 



carriage, and fix them on immovable objects, such as the sides of the com- 

 partment, or the faces of our travelling companions, the images on the retina 

 will really preserve the same size, and yet the objects will appear larger. 

 Such are some of the interesting facts among those discovered by Mr. 

 Thompson; and though we do not intend to push the inquiry further, we 

 think it may not be without interest to describe here another illusion of that 

 organ whose properties are in every way so curious and remarkable. 



Fig. 139. Design for experiment or the pu,::inm ca-cunt. 



Another experiment to show the existence of impressions received by 

 the retina can be made with the figure above (138). If the gaze be fixed upon 

 the dark spot in the centre of the white figure for about half a minute, and 

 the eyes then directed to the ceiling, or a sheet of white paper, the white 

 figure will be reproduced in black. This result is based upon the principle 

 of complementary colours. A red design, for instance, will be reproduced 

 in green. 



