262 SUPPLEMENT TO THE 



little time the effect wears off, and we again see it in bas- 

 relief; then again in alt; and so on, by successive fits. 

 This deception arises from the simple circumstance that 

 the lights and shades in bas-relief are very nearly like those 

 of an alto-relievo of the same subject, illuminated from the 

 opposite side ; our understanding in this case instantly 

 corrects the false testimony of the eye, when we consider 

 from which side the light comes. (If we observe with an 

 engiscope, we must always remember that its image is 

 inverted, and that in consequence the light must be con- 

 sidered as proceeding from the side of the field of view 

 opposite to that where the source of illumination actually 

 exists.) It will also be highly advisable, when we are in 

 doubt as to the manner in which an instrument shews 

 prominences and depressions, to verify its vision by ob- 

 serving some known object with it, of the real state of 

 which, as to inequality of surface, we have been previously 

 informed by the sense of touch, to which it has been well 

 said there is no fellow.* 



19. Illumination, by cups or silver specula, does not 

 produce these illusions, because they create no shade the 

 whole object is one mass of intense light ; other false per- 

 ceptions are, however, occasioned by them. Thus, all 

 globular bodies, having polished surfaces, reflect an image 

 of the cups, and the pout, if there is one, appears as a 

 dark spot in the centre. The eyes of insects, illuminated 

 in this way, shew the semblance of a pupil in the centre of 

 each lens, which deception may be verified by examining 



* We usually see objects illuminated from above with the shadows below 

 the prominences ; now, unless the light is below an opaque object, when we 

 view it in an engiscope, we shall see the shadows above, giving the promi- 

 nences the appearance of depressions, and producing a very unnatural effect. 

 A. P. 



