[ 432 ] 



LXVII. On the Cotwersion of Relief by Inverted Vision. Bi/ 

 Sir David Brewster, K.H., D.C.L., F.R.S., and V.P.R.S. 



Edin* 

 TTNDER the name Conversion of Relief an expression first 

 ^ used by Mr. Wheatstone, I include all those optical 

 illusions which take place in the vision of cameos and inta- 

 glios, of elevations and depressions, whether they are produced 

 with opake'or transparent bodies, — no surfaces with or without 

 shadows, — in reflected or transmitted light, — while using one 

 or both eyes, —or by erect or inverted vision. In these various 

 forms of the pheenomenon, the illusion is modified by certain 

 secondary causes, which were regaided both by Mr. Wheat- 

 stone t and my self J as primary causes; so that we were led 

 away, each in a different direction, from the right path of 

 inquiry. 



The phasnomenon occurs in its most general and simple 

 form, when it is produced by viewing a shadowless depression, 

 or elevation, made in an extended surface, through an invert-, 

 ing microscope, or the inverting eye-piece of a telescope, and 

 at an angle intermediate between 0° and 90°. In so far as I 

 know, the pheenomenon has never been thus limited, and, 

 consequently, no explanation of it has ever been given. That 

 which I shall now submit to the Society is capable of the most 

 rigorous demonstration ; and when it is once in our possession, 

 we can have no difficulty in recognising the secondary causes 

 which increase or diminish the influence of the primary one, 

 and which, in its absence, are sometimes the immediate cause 

 of the illusion. 



Let A, fig. 1, be a deep spherical concavity, and A', fig. 2, 



Fig. 1. Fig. 2. 



a high spherical convexity in an extended horizontal {table 

 M N, M' N', and let them be shadowless, or illuminated by a 



^ * Read at the Royal Society of Edinburgh, May 6, 1844. See tlieir 

 Transactions, vol. xv. p. 657. 



t Philosophical Transactions, 1838, pp. 383, 384. 

 X Edinburgh Transactions, vol. xv. p. 365; Edinburgh Journal of ScieDce, 

 vol. iv. p. 97 J and Letters on Natural Magic, p. 98. 



