Dr. Young's Lecture 
44 
observable in examining the lens after death. The situation of 
the rays is constant, though not regular ; the most conspicuous 
are seven or eight in number ; sometimes about twenty fainter 
ones may be counted. Removing the point a little further, the 
image becomes a short vertical line ; the rays that diverged ho- 
rizontally being perfectly collected, while the vertical rays are 
still separate. In the next stage, which is the most perfect 
focus, the line spreads in the middle, and approaches nearly to 
a square, with projecting angles, but is marked with some 
darker lines towards the diagonals. The square then flattens 
into a rhombus, and the rhombus into a horizontal line un- 
equally bright. At every greater distance, the line lengthens, 
and acquires also breadth, by radiations shooting out from it, but 
does not become a uniform surface, the central part remaining 
always considerably brightest, in consequence of the same flat- 
tening of the vertex which before made it fainter. Some of 
these figures bear a considerable analogy to the images derived 
from the refraction of oblique rays, (Schol. 4. Prop. IV.) and 
still more strongly resemble a combination of two of them in 
opposite directions ; so as to leave no doubt, but that both sur- 
faces of the lens are oblique to the visual axis, and co-operate 
in distorting the focal point. This may also be verified, by 
observing the image delineated by a common glass lens, when 
inclined to the incident rays. (See Plate VI. Fig. 28 — 40.) 
The visual axis being fixed in any direction, I can at the 
same time see a luminous object placed laterally at a consi- 
derable distance from it; but in various directions the angle 
is very different. Upwards it extends to 50 degrees, inwards 
to 60, downwards to 70, and outwards to go degrees. These 
internal limits of the field of view nearly correspond with 
