iBo Mr. Young's Observations on Vision. 
nolds has elegantly insisted on in his discourses ; so that per- 
haps some principles of beautiful contrast of colours may be 
drawn from hence, it being probable that those colours which 
together approach near to white light will have the most 
pleasing effect in apposit on. It must be observed, that the 
sensation of light from pressure of the eye subsides almost in- 
stantly after the motion of pressure has ceased, so that the 
cause of the irritation of the retina is a change, and not a dif- 
ference, of form ; and therefore the sensation of light appears 
to depend immediately on a minute motion of some part of 
the optic nerve. 
If the anterior part of the eye be repeatedly pressed, so as 
to occasion some degree of pain, and a continued pressure be 
then made on the sclerotica, while an interrupted pressure is 
made on the cornea ; we shall frequently be able to observe 
an appearance of luminous lines, branched, and somewhat con- 
nected with each other, darting from every part of the field of 
view, towards a centre a little exterior and superior to the axis 
of the eye. This centre corresponds to the insertion of the 
optic nerve, and the appearance of lines is probably occasioned 
by that motion of the retina which is produced by the sudden 
return of the circulating fluid, into the veins accompanying 
the ramifications of the arteria centralis, after having been 
detained by the pressure which is now intermitted. As such 
an obstruction and such a re-admission must require particu- 
lar circumstances, in order to be effected in a sensible degree, 
it may naturally be supposed that this experiment will not 
always easily succeed. 
