IN SINGLE AND BINOCULAR VISION. 365 



to investigate this subject, and in January 1826 I published an account of my ob- 

 servations, with an ample notice of the previous labours of other authors.* 



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 fol- 

 lowing observations upon it. " These considerations do not fully explain the phe- 

 nomenon, 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 in- 

 vert it, and also when it is equally illuminated in all parts."-j- In thus objecting 

 to the fulness of my explanation, Mr WHEATSTONE has overlooked the great num- 

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

 which I observed the fallacy when the object is viewed without 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 shew that the projection of either on 

 the retina is sensibly the same4 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 judgment, viz., the 

 presentation of a different picture to each eye, is wanting ; the imagination there- 

 fore 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 judgment 

 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 pheno- 

 menon is to be found in the indetermination of the judgment, arising from our more 

 perfect means of judging being absent." $ 



Now, what Mr WHEATSTONE calls the real cause of the illusion is no cause at 

 all, it is merely a previous state of the mind which is favourable to the opera- 

 tion of the real cause. Two eyes, like two witnesses, must always bear a better 

 testimony to truth, than one ; and, in the present case, the want of the conver- 

 gency of the optic axes to estimate the distance of the highest and lowest points 

 of the cameo and the intaglio, undoubtedly favours the illusion, and allows the 

 real cause to influence the judgment ; but even here this admission has its limits, 



* 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. 



f Phil. Trans. 1838, p. 383. 



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



Phil. Trans. 1838, p. 384. 



VOL. XV. PART III. 5 F 



