256 Prof. Wheatstone on the Physiology of Vision. 



These considerations do not fully explain the phsenomenon, 

 for they suppose that the image must be inverted, and that the 

 light must fall in a particular direction ; but the conversion of 

 relief will still take place when the object is viewed through an 

 open tube without any lenses to invert it, and also when it is 

 equally illuminated in all parts. The true explanation I believe 

 to be the following. If we suppose a cameo and an intaglio of 

 the same object, the elevations of the one corresponding exactly 

 to the depressions of the other, it is easy to show that the pro- 

 jection of either on the retina is sensibly the same. When the 

 cameo or the intaglio is seen with both eyes, it is impossible to 

 mistake an elevation for a depression, for reasons which have 

 been already amply explained ; but when either is seen with one 

 eye only, the most certain guide of our judgement, viz. the pre- 

 sentation of a different picture to each eye, is wanting ; the 

 imagination therefore supplies the deficiency, and we conceive 

 the object to be raised or depressed according to the dictates of 

 this faculty. No doubt in such cases our judgement is in a great 

 degree influenced by accessory circumstances, and the intaglio 

 or the relief may sometimes present itself according to our pre- 

 vious knowledge of the direction in which the shadows ought to 

 appear ; but the principal cause of the phenomenon is to be 

 found in the indetertnination of the judgement arising from 

 our more perfect means of judging being absent. 



Observers with the microscope must be particularly on their 

 guard against illusions of this kind. Raspail observes * that 

 the hollow pyramidal arrangement of the crystals of muriate of 

 soda appears, when seen through a microscope, like a striated 

 pyramid in relief. He recommends two modes of correcting the 

 illusion. The first is to bring successively to the focus of the 

 instrument the different parts of the crystal ; if the pyramid be 

 in relief, the point will arrive at the focus sooner than the base 

 will; if the pyramid be hollow, the contrary will take place. 

 The second mode is to project a strong light on the pyramid in 

 the field of view of the microscope, and to observe which sides of 

 the crystal are illuminated, taking however the inversion of the 

 image into consideration if a compound microscope be employed. 



The inversion of relief is very striking when a skeleton cube 

 is looked at with one eye, and the following singular results may 

 in this case be observed. So long as the mind perceives the 

 cube, however the figure be turned about, its various appear- 

 ances will be but different representations of the same object, 

 and the same primitive form will be suggested to the mind by 

 all of them : but it is not so if the converse figure fixes the at- 



* Nouveau Systhne de Chimie Organique, 2 me edit. t. i. p. 333. 



