282 
NEAR AND DISTANT SIGHT. 
Facts relating ISl I, has taken pains to ascertain whether the power by which 
ofdHatati^on^of adjusted to see at different distances, depends, in 
the pupil, &c. any degree, on the faculty in the pupil of dilating and con- 
tracting ; and whether its fixed dilatation has any influence in 
preventing an accurate view of near objects. This last men- 
tioned eflect Dr. Wells relates to have taken place remarkably 
in the case of Dr. Cutting, whose pupil being fixed in a dilated 
state by the action of the extract of belladonna, perfect vision 
of a near object was removed, as the dilatation advanced, from 
six inches (which was the nearest distance at which Dr. Cut- 
ting could distinctly see the image of the flame of a candle 
reflected from the bulb of a small thermometer) to seven 
inches in thirty minutes, and to three feet and a half in three 
quarters of an hour. My eldest son, who has a very extensive 
range of vision, has made a similar experiment on his right 
eye with a similar result. Previous to the application of the 
belladonna, he could bring the apparent lines on an optometer 
(like that improved by Dr. Young from the invention of 
Dr. Porterfield, and described in the Philosophical Transactions 
for the year 1800 ) to meet at four inches from the eye j and, 
by directing his attention to a more distant point, he could 
prevent them from meeting till they were seven inches from 
the eye, after wliich they continued apparently united the 
\i hole length of the optometer, which was twelve inches*. He 
could see the iir.age of a candle reflected from the bulb of a 
small ihermometcr, five-sixteenths of an inch in diameter, at 
the d;sta>ice of three inches and three quarters from the eye j 
and he couid also see the image at the distance of two feet 
seven inches. The belladonna produced a conspicuous dila- 
tation of the pupil in less than an hour j after which, on view- 
ing the apparent lines on the optometer, he was unable to 
• Tlie tHo lines that are perceived on looking throuch the slits of 
an optometer, cross each other precisely in the point from whence the 
rajs of light diverge in order to be brought to a focus on the retina. 
And their apparent union, before and after Ibis point, is occasioned 
by the unavoidable thickness of Ute line drawn on the optometer. 
make . 
