88 VISION. 
universe? Is it philosophical to suppose that there 
is no other universe than that which is indicated to 
us by whirling spheres that roll through space, 
and that these must be shorn even of those pro- 
perties by which they directly affect our senses, 
the properties by which alone we have acquired 
knowledge of their existence? It is easy to see 
what strange possibilities may be opened up by 
such a question, but that is no reason why the 
question should not be asked. Either we must 
suppose that the sensations of sight and sound 
have links in some way corresponding to them in 
a world of which we know nothing, or that they 
are arbitrary illusions, graduated doubtless in keep- 
ing with movements not directly appreciable by us, 
but arbitrary nevertheless. 
