308 
Peaucillier’s motion. 
BECD is a rhombus formed of equal rods jointed by pins 
at B, D, C, H. It is jointed by pins at Band C to two equal 
rods AB, AC, which are moveable about a fixed pin at A. 
It is manifest that the straight line A # makes equal angles 
with ZB and EC, and will therefore if produced pass through D. 
It is also manifest that 
ADx AEF=AM — HEM? = AC*— EC? =a constant. 
Hence whatever curve is traced out by the point H, say 
p= (@), the curve traced out by D will be ack (0), when 
p=AE and p=AD. Hence, if His made to describe a circle 
passing through A, D will describe a straight line. # is made 
to describe this circle by means of a bridle-rod KE (= KA) 
moveable round a fixed pin at K. 
March 2, 1874. 
THE PRESIDENT (PROFESSOR BABINGTON) in the Chair. 
On the Relation of Geometrical Optics to other parts 
of Mathematics and Physics. By Prof. Cimrx 
MAXWELL. 
The study of geometrical optics may be made more in- 
teresting to the mathematician by treating the relation between 
