35 
on the Mechanism of the Eye. 
each point of the line, except the first point (R, Fig. 6,) will 
have a double image ; and each pair of images, being contiguous 
to those of the neighbouring radiant points, will form with them 
two continued lines, and the images being more widely sepa- 
rated as the point which they represent is further from the first 
radiant point, the lines ( st,st ,) will converge on each side 
towards (r) the image of this point, and there will intersect 
each other. 
The same happens when we look at any object through two 
pin holes, within the limits of the pupil. If the object be at the 
point of perfect vision, the image on the retina will be single ; 
but, in every other case, the image being double, we shall appear 
to see a double object: and, if we look at a line pointed nearly 
to the eye, it will appear as two lines, crossing each other in the 
point of perfect vision. For this purpose, the holes may be 
converted into slits, which render the images nearly as distinct, 
at the same time that they admit more light. The number may 
be increased from two to four, or more, whenever particular 
investigations render it necessary. 
The optometer may be made of a slip of card-paper, or of 
ivory, about eight inches in length, and one in breadth, divided 
longitudinally by a black line, which must not be too strong. 
The end of the card must be cut as is shown in Plate III. Fig 7, 
in order that it may be turned up, and fixed in an inclined 
position by means of the shoulders : or a detached piece, nearly 
of this form, may be applied to the optometer, as it is here en- 
graved. A hole about half an inch square must be made in this 
part ; and the sides so cut as to receive a slider of thick paper, 
with slits of different sizes, from a fortieth to a tenth of an inch 
in breadth, divided by spaces somewhat broader; so that each 
observer may choose that which best suits the aperture of his pupil. 
F 2 
