PROCEEDINGS OF SOCIETIES. 133 
obliterate the other; it sees, in fact, whichever it wills 
to see, and the other image, simply by being neglected, 
becomes invisible. In ordinary vision, the determination of 
the image to be seen is effected by the same act of the will 
which determines the position of the optic axes; but by 
certain arrangements which were indicated both images may 
be made to have the same relation to the optic axes; and as 
the predisposition to select one or the other is thus obviated, 
it is made indifferent to the mind which of the two images 
that occupy the same place in the sensoriwm it shall see. 
When these arrangements are made, it is found that mere 
efforts of the will can easily bring either the one or the other 
into view. The importance of the law, which enables the 
mind to select its image, was pointed out in different cases of 
ordinary vision. It obviates the difficulty already adverted 
to, of haying two different pictures on the same spot; it has 
not improbably an important influence in producing the 
general stereoscopic effect ; it also, to some extent, remedies 
the effect of squinting, by obliterating the picture in the 
imperfect eye, which could not be else done without shutting 
it. The effect of the law, in some extraordinary cases, was 
also noticed, especially in the power of the will to fix images 
on the sight, as Sir Isaac Newton instances in his own 
ease (see his ‘ Life,’ by Sir David Brewster). The author 
pointed out the great interest of the subject, not only in its 
practical aspect, but also as having an important bearmg on 
the connexion between mind and matter. 

Dustin University ZooLtoGicaL AND BoTaNIcAL 
ASSOCIATION. 
April 16th, 1858. 
Supplementary Catalogue of Desmidiacee found in the neigh- 
bourhood of Dublin, with Description and Figures of a 
proposed New Genus, ang of Four New Species. By 
Wiiram Arcuer, M.R.D.S. 
Class.—ALG i. 
Order.—CHLoRosPORE or CoNFERVOIDER. 
Family.—DesMmipiace2. 
LEPTOCYSTINEMA, 0. g. 
Plant an elongated jointed filament (often separating) ; 
joints straight, much elongated and slender, without a 
