384 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



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 objec^, the elevations of the one corresponding exactly 

 to the depressions of the other, it is easy to show that the projection 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 presentation of a different picture to each eye, is 

 wanting; the imagination therefore supplies the deficiency, and we conceive the ob- 

 ject 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 previous 

 knowledge of the direction in which the shadows ought to appear ; but the real cause 

 of the phenomenon is to be found in the indetermination 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 primi- 

 tive form will be suggested to the mind by all of them : but it is not so if the con- 

 verse figure fixes the attention ; the series of successive projections cannot then be 

 referred to any figure to which they are all common, and the skeleton figure will ap- 

 pear to be continually undergoing a change of shape. 



§ 12. 



I have given ample proof that objects whose pictures do not fall on corresponding 

 points of the two retinae may still appear single. I will now adduce an experiment 

 which proves that similar pictures falling on corresponding points of the two retinee 

 may appear double and in different places. 



Present, in the stereoscope, to the right eye a vertical line, and to the left eye a 



* Nouveau Syst^me de Chimie Organique, 2™* edit. t. 1. p. 333. 



