Dr. Young’s Lecture 
36 
In order to adapt the instrument to the use of presbyopic eyes, 
the other end must be furnished with a lens of four inches focal 
length ; and a scale must be made near the line on each side 
of it, divided from one end into inches, and from the other ac- 
cording to the table here calculated from Cor. 7. Prop. IV, by 
means of which, not only diverging, but also parallel and con- 
verging rays from the lens are referred to their virtual focus. 
The instrument is easily applicable to the purpose of ascer- 
taining the focal length of spectacles required for myopic or 
presbyopic eyes. Mr. Cary has been so good as to furnish 
me with the numbers and focal lengths of the glasses com- 
monly made ; and I have calculated the distances at which those 
numbers must be placed on the scale of the optometer, so that 
a presbyopic eye may be enabled to see at eight inches distance, 
by using the glasses of the focal length placed opposite to the 
nearest crossing of the lines ; and a myopic eye with parallel 
rays, by using the glasses indicated by the number that stands 
opposite their furthest crossing. To facilitate the observation, 
I have also placed these numbers opposite that point which 
will be the nearest crossing to myopic eyes ; but this, upon the 
arbitrary supposition of an equal capability of change of focus 
in every eye, which I must confess is often far from the truth. 
It cannot be expected, that every person, on the first trial, 
will fix precisely upon that power which best suits the defect 
of his sight. Few can bring their eyes at pleasure to the state 
of full action, or of perfect relaxation; and a power two or 
three degrees lower than that which is thus ascertained, will be 
found sufficient for ordinary purposes. I have also added to the 
second table, such numbers as will point out the spectacles 
necessary for a presbyopic eye, to see at twelve and at eighteen 
inches respectively : the middle series will perhaps be the most 
