43 
on the Mechanism of the Eye. 
These considerations will explain the inconsistency of dif- 
ferent observations on the refractive power of the crystalline ; 
and, in particular, how the refraction which I formerly calcu- 
lated, from measuring the focal length of the lens,* is so much 
greater than that which is determined by other means. But, 
for direct experiments. Dr. Wollaston's method is exceed- 
ingly accurate. 
When I look at a minute lucid point, such as the image of 
a candle in a small concave speculum, it appears as a radiated 
star, as a cross, or as an unequal line, and never as a perfect 
point, unless I apply a concave lens inclined at a proper angle, 
to correct the unequal refraction of my eye. If I bring the 
point very near, it spreads into a surface nearly circular, and 
almost equably illuminated, except some faint lines, nearly in a 
radiating direction. For this purpose, the best image is a can- 
dle, or a small speculum, viewed through a minute lens at some 
little distance, or seen by reflection in a larger lens. If any 
pressure has been applied to the eye, such as that of the finger 
keeping it shut, the sight is often confused for a short time after 
the removal of the finger, and the image is in this case spotty 
or curdled. The radiating lines are probably occasioned by 
some slight inequalities in the surface of the lens, which is very 
superficially furrowed in the direction of its fibres : the curdled 
appearance will be explained hereafter. When the point is fur- 
ther removed, the image becomes evidently oval, the vertical 
diameter being longest, and the lines a little more distinct than 
before, the light being strongest in the neighbourhood of the 
centre ; but immediately at the centre there is a darker spot, 
owing to such a slight depression at the vertex as is often 
* Phil. Trans, for 1793. p. 174. 
G 2 
