in Single and Binocular Vision, 451 



7, On the Vision of Cameos and Intaglios*. 



The beautiful experiment of converting a cameo into an in- 

 taglio, and an intaglio into a cameo, by monocular vision, is 

 well known. In 1825 I had occasion to investigate this sub- 

 ject, and in January 1826 I published an account of my ob- 

 servations, with an ample notice of the previous labours of 

 other authors f. 



Mr. Wheatstone has ingeniously connected this optical 

 fallacy with the union of dissimilar images on the retina, though 

 he does not refer it to this union as its cause. After quoting 

 my previous explanation of the illusion, he makes the follow- 

 ing observations upon it: — "These considerations do not fully 

 explain the phenomenon, for they suppose that the image must 

 be inverted, and that the light must fall in a particular direc- 

 tion ; 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 %." 

 In thus objecting to the fullness of my explanation, Mr. 

 Wheatstone has overlooked the great number of experiments 

 by which I have supported it; and especially those facts in 

 which I observed the fallacy when the object is viewed with- 

 out even an open tube, — without inversion ; — with both eyes open, 

 and when it is placed in broad daylight. Mr. Wheatstone 

 then gives his own opinion as follows : — " 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 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 ; but when either is seen by one eye only, the most 

 certain guide of our judgement, viz. the presentation of a differ- 

 ent 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 influ- 

 enced by accessory circumstances, and the intaglio or the relief 

 may sometimes present itself according to our previous know- 

 ledge of the direction in which the shadows ought to appear; 

 but the real cause of the phaenomenon is to be found in the 



* The true explanation of this class of phenomena will be given in an 

 early Number. — D. B. 



f This account was published anonymously in the Edinburgh Journal of 

 Science for January 1826, No. VII. vol. iv. p. 97; and a popular abstract 

 of it afterwards appeared in my Letters on Natural Magic, Letter V. p. 98. 



% Philosophical Transactions, 1838, p. 383. 



§ This is true only when they are not seen obliquely. — D. B. 



2 G2 



