160 _ &. Rowley on Vision. 
But to make the accurate performance of these experiments — 
easy, some special instrument must be provided. <A piece of — 
pine board whose opposite great surfaces are rectangles paral- 
lel to one another, thirteen inches long and eight inches wide, 
with five pins, furnishes the means for obtaining accurate Te 
sults, where the distance between the near and far points does — 
not exceed twelve inches.* | 
- 
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a 
a 
oO 
ro 
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a “al 
of the eyes, and, together with the pin at B, in the verut” 
plane which bisects this line at right angles ; which ¢ the 
__ ton of the pins is had, when, looking at the images”. 
pin at A, the left eye’s image of the pin at T’, and the right 
and at a point a little beyond where the latter is see; © 
Wann Ose are fixed upon the images of the pin at B, the 
eyes image of that at A will 
* Another board of the same width and 18 inches long serving aS co pie 
tis, to be held with one end against the nose by the hands pressi0g 
at the sides, will be found serviceable for maintaining steadiness of POST” 
7 
