1888. ] and on a Photo-Voltaic Theory of Vision. 313 
action is photo-voltaic; both a transitory E. M. F. opposed to the 
“normal” current, and a persistent diminution of resistance being 
produced. The physiological conditions however were necessarily 
complex, so these explanations are not given in detail, as it 
would perhaps be explaining too much. 
On the whole the most striking analogy between vision and 
photo-voltaic action is the range of colour which produces the 
action in each case. Supposing vision to be due to photo- 
chemical action, it has always been a paradox to see why the 
less refrangible end of the spectrum should be visible as far as 
the red, while the powerfully actinic rays beyond the violet are 
almost entirely non-luminous. 
(6) On the Geometrical Interpretation of the Singular Points 
of an Equipotential System of Curves. By J. Briti, M.A., St 
John’s College. 
1. Ina paper presented to the Society during the Lent Term 
I called attention to a paper published by Siebeck in the fifty-fifth 
volume of Crelle. In that paper he enunciates a theorem which 
is equivalent to the statement that all equipotential systems of 
curves are confocal, the branch points of the system being the 
common foci of the constituent curves of the system. This 
statement, as I pointed out, is contradicted by the well-known 
case of a system of coaxal circles. By studying this case I have 
been enabled to find out where Siebeck went wrong*, and also 
to obtain a geometrical interpretation of the branch points of 
an equipotential system of algebraical curves. And, in what 
follows, I have also shewn that this interpretation gives promise 
of capability of development into a method which would enable 
us to discover all the possible equipotential families consisting of 
algebraical curves of a given order. 
2. In seeking the intersections of real algebraical curves 
imaginary values turn up in pairs. ‘Thus if 
x=a+b, y=ct+id 
be the coordinates of one of the points of intersection, there will 
be a corresponding point of intersection whose coordinates are 
x=a—1b, y=c-—id. 
We shall call these points respectively A and B. We shall also 
denote by the symbol J that circular point at infinity that hes on 
the line #+7y =0, and by the symbol J that one that lies on 
the line —iy=0. Then the equations of the lines 
* Kummer appears to have fallen into an error similar to that of Siebeck: 
see a paper in the thirty-fifth volume of Crelle, entitled ‘‘ Ueber Systeme von 
Curven welche einander iiberall rechtwinklig durchschneiden.” 
