Mr. T. S. Davies on Geometry and Geometers. 523 



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 ob- 

 served similar effects with Daguerreotypes of medallions and 

 cameos, and with carefully shaded drawings of simple objects. 



LXXI. Geometry and Geometers. Collected by the late Thomas 

 Stephens Davies, F.R.S.L.tyE.fyc* 



No. X. 



[Continued from p. 290.] 



THERE is another ground of embarrassment to the young 

 mathematician in forming his estimate of the ancient geo- 

 metry. It is the want of proper discrimination between classes 

 of propositions which are in themselves of essentially distinct 

 characters. This is traceable to our very elements ; for even the 

 first three books of Euclid comprise indiscriminately almost 

 every kind of proposition — determinate and indeterminate. I 

 need only refer to Mr. Potts' s " Appendix/' before referred to 

 (p. 289), for proof of this; for it will there be seen how di- 

 versified are the propositions as to logical character, which con- 



* Communicated by James Cockle, Esq., M.A., Barrister-at-Law, who 

 adds the following note : — 



[" The above autograph of the late Professor Davies (for this addition 

 to which I am responsible) constitutes the residue of the paper of which the 

 remaining portion appeared in the April Number of this Journal. I have 

 now communicated to the Philosophical Magazine for publication all the 

 manuscripts of my ' f c friend, which Mrs. Davies has confided to me. But 

 I have no doubt that, in the ample stoic which I believe still remains in her 

 hands, much will be found of the working of his genius — much that, while 

 it reminds science of the loss she has sustained, will render important ad- 

 vantages to mathematical literature, and prove worthy of the name and re- 

 putation of the departed philosopher. 



"James Cockle. 



" 2 Pump Court, Temple, 

 May 11, 1B52."] 



