54 
Dr. Young’s Lecture 
1 795,* appears to be far preferable to the apparatus of the 
preceding year : -f for a difference in the distance of two images 
seen in the cornea, would be far greater, and more conspicuous, 
than a change of its prominency, and far less liable to be 
disturbed by accidental causes. It is nearly, and perhaps 
totally impossible to change the focus of the eye, without some 
motion of its axis. The eyes sympathize perfectly with each 
other ; and the change of focus is almost inseparable from a 
change of the relative situation of the optic axes ; so much, that 
if I direct both my eyes at an object beyond their furthest focus, 
I cannot avoid bringing that focus a little nearer : while one 
axis moves, it is not easy to keep the other perfectly at rest ; 
and it is not impossible, that a change in the proportions of some 
eyes, may render a slight alteration of the position of the axis 
absolutely necessary. These considerations may partly explain 
the trifling difference in the place of the cornea that was ob- 
served in 1794,. It appears that the experiments of 1795 were 
made with considerable accuracy, and no doubt with excellent 
instruments ; and their failing to ascertain the existence of any 
change, induced Mr. Home and Mr. Ramsden to abandon, 
in great measure, the opinion which suggested them, and to 
suppose, that a change of the cornea produces only one-third of 
the effect. Dr. Olbers of Bremen, who in the year 1780 
published a most elaborate dissertation on the internal changes 
of the eye, J which he lately presented to the Royal Society, 
had been equally unsuccessful in his attempts to measure this 
change of the cornea, at the same time that his opinion was in 
favour of its existence. 
• Phil. Trans, for 1796, p. 2. f Phil. Trans, for 1795, p. 13. 
X De Oculi Mutationibus internis. Gotting. 1780. 4 0 , 
