6(50 SIR DAVID BREWSTER ON THE 



looking at its image in the focus of a convex lens, it will sometimes appear a con- 

 vexity, and sometimes not. In this form of the experiment the image of the con- 

 cavity, and consequently its apparent depth, is greatly diminished. Hence any 

 trivial cause, such as a preconception of the mind, or an approximation to a 

 shadow, or a touch of the hollow by the point of the finger, will either produce a 

 conversion, or prevent it. 



In the preceding experiments we have supposed the convexity to be high and 

 the concavity deep and circular, and we have supposed them also to be shadow- 

 less, or illuminated by a quaquaversus light, such as that of the sky in the open 

 fields. This was done to get rid of all secondary causes, which interfere with and 

 modify the normal cause when the concavities and convexities are shallow, and 

 have distinct shadows, or when the concavity has the shape of an animal, or any 

 body which we are accustomed to see convex. 



Let us now suppose that a strong shadow is thrown upon the concavity. In 

 this case the normal experiment, already explained and shewn in Fig. 5, is much 

 more perfect and satisfactory. The illusion is complete, and invariable when the 

 concavity is in an extended surface ; and it as invariably disappears when it is 

 in a narrow stripe. 



In the secondary forms of the experiment, the inversion of the shadow 

 becomes the principal cause of the illusion ; but, in order that the result may be 

 invariable, or nearly so, the concavities must be shallow, and the convexities a 

 little raised. At great obliquities, however, this cause of the conversion of Form 

 ceases to produce the illusion, and in varying the inclination from to 90, the 

 cessation takes place sooner with deep than with shallow cavities. The reason of 

 this is, that the shadow of a concavity is very different at great obliquities from 

 the shadow of a similar convexity. The shadow never can emerge out of a cavity 

 so as to darken the surface in which the cavity is made ; whereas the shadow of 

 a convexity soon extends beyond the outline of its base, and, finally, throws a long 

 stripe of darkness over the surface on which it rests. Hence it is impossible 

 to mistake a convexity for a concavity, whenever its shadow extends beyond its 

 base. 



When the concavity is a horse or a dog upon a seal, it will often rise into a 

 convexity when seen through a single lens which does not invert it ; but the illu- 

 sion disappears at great obliquities. In this case the illusion is favoured, or pro- 

 duced, by two causes : the first is, that the convex form of the horse or dog is 

 the one which the mind is most disposed to seize ; and the second is, that we use 

 only one eye, with which we cannot measure depths as well as with two. The 

 illusion, however, still takes place when we employ a lens three or more inches 

 wide, so as to admit the use of both eyes, but it is less certain, as the binocular 

 vision enables us to keep in check, to a certain degree, the other causes of illusion. 



The influence of these secondary causes is strikingly displayed in the follow- 



