MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
17 
^ 24 . 
The cases of the conversion of relief when the object is reg-ardecl with one eye only, 
some of which were known more than a century ago, were taken into consideration 
and endeavoured to be explained by me in § 11 of the first part of this memoir, and 
Sir David Brewster* has published some interesting and instructive observations on 
the same subject; I will therefore not revert to this matter here, but only to say that 
I have myself never observed the conversion of relief when looking with both eyes 
immediately on a solid object, and if it has been observed by others under such 
circumstances, I should be inclined to attribute the effect to an inequality in the- im- 
pressions on the two eyes so that one only is attended to. But the plane shaded 
representation of a solid object, the relief of which is not very deep, may easily be 
made to appear at will either as the solid which it is intended to represent or as its 
converse, even when both eyes are employed. This effect is strikingly observed in 
the glyptographic engravings of medals of low relief, and depends entirely on whether 
the light is so placed that it would cast the same shadows on the real object as are 
represented in the picture, or that it would cast shadows in the opposite direction. 
In the former case the picture appears with the relief it was intended to suggest ; in 
the latter with the converse relief. I have observed similar effects with Daguerreotypes 
of medallions and cameos, and with carefully shaded drawings of simple objects. 
* Transactions of the Royal Society of Edinburgh, vol. xv. p. 365 and 657. 
MDCCCLII, 
D 
