OPTICAL ILLUSIONS. 



117 



The same illusion is presented in a number of examples in the course 

 of daily life. An empty room appears smaller than a furnished room, and 

 a wall covered with paperhangings appears larger than a bare wall. It is a 

 well-known source of amusement to present someone in company with a 

 hat, and request him to mark on the wall its supposed height from the 

 ground. The height generally indicated will be a size and a half too large. 



We will relate an experience described by Bravais : " When at sea," 

 he says, " at a certain distance from a coast which presents many inequalities, 

 if we attempt to draw the coastline as it presents itself to the eye, we shall 

 find on verification that the horizontal dimensions have been correctly 

 sketched at a certain scale, while all the vertical angular objects have been 

 represented on a scale twice as large. This illusion, which is sure to occur 

 in estimates of this kind, can be demonstrated by numerous observations." 



M. Helmholtz has also indicated several optical illusions. 



Fig. 117. 



Fig. 119. 



Fig. 1 1 8. 



If we examine fig. 1 1 8, the continuation of the line a does not 

 appear to be d, which it is in reality, but f t which is a little lower. This 

 illusion is still more striking when we make the figure on a smaller scale 

 (fig. 1 1 9), as at B, where the two fine lines are in continuation with each 

 other, but do not appear to be so, and at c, where they appear so, but are 

 not in reality. If we draw the figures as at A (fig. 1 1 8), leaving out the line 

 d y and look at them from a gradually increasing distance, so that they appear 

 to diminish, it will be found that the further off the figure is placed, the 

 more it seems necessary to lower the line fto make it appear a continuation 

 of a. These effects are produced by irradiation ; they can also be produced 

 by black lines on a white foundation. Near the point of the two acute 

 angles, the circles of diffusion of the two black lines touch and mutually 

 reinforce each other ; consequently the retinal image of the narrow line 

 presents its maximum of darkness nearest to the broad line, and appears 

 to deviate on that side. In figures of this kind, however, executed on a 

 larger scale, as in fig. 1 1 8, irradiation can scarcely be the only cause of 



