4 The Realm of Nature CHAP. 



smell and taste are limited in their scope, and liable to get 

 out of order through disease or neglect. But even when 

 in full health and within their own range they are not fully 

 trustworthy. If an object present different appearances 

 when looked at through different windows, we are justified 

 in supposing that the windows are not equally trustworthy. 

 A few simple experiments show us that this is the case with 

 the windows of knowledge. Every one is familiar with 

 optical illusions proving the imperfection of the sense of 

 sight. A coin spinning quickly looks like a hazy sphere, 

 but we know it to be a flat disc. Strobic circles which seem 

 to whirl rapidly when the card on which they are printed 

 is moved slightly, and designs appearing in their com- 

 plementary colours on looking at a blank wall have been 

 made familiar by their use as advertisements. Mountains 

 always look steeper than they really are ; in a slight haze 

 on a wide moor a rabbit close at hand may be mistaken for 

 a distant deer, and the most familiar object is often passed 

 unrecognised if in an unusual place. One well-known 

 experiment shows that touch is as fallacious as sight. When 

 a pea or small ball is rolled on a table by the middle finger 

 crossed over the forefinger of the same hand, so that both 

 fingers touch the object, the impression produced is that 

 there are two peas, not one. Similarly if one hand has been 

 held in hot water and the other in cold water, and then 

 both are plunged into a mixture of hot and cold, the 

 mixture will be pronounced cold by the heated hand and 

 hot by the chilled one. The deceitfulness of the senses 

 may impose upon the most acute and practised mind if 

 taken unawares. When Sir Humphry Davy discovered 

 potassium he showed a piece of it to Dr. Wollaston, one of 

 the most accurate observers who ever lived. Wollaston 

 saw the silvery lustre of the new metal, weighed it in his 

 hand and said, " How ponderous it is ! " Davy in reply 

 threw the metal into a basin of water, where it floated 

 lightly on the surface. Wollaston's illusion is the more 

 striking because at that time he was the only man who 

 was in the habit of handling platinum, a metal which, bulk 

 for bulk, is twenty-five times heavier than potassium. 



