268 Mr. Airy on a peculiar Defect in the Eye. 
in my left eye, was not circular, as it is in the eye which has no 
other defect than that of being near-sighted, but elliptical, the 
major axis making an angle of about 35° with the vertical, and 
its higher extremity being inclined to the right. Upon putting 
on concave spectacles, by the assistance of which I saw distant 
objects distinctly with my right eye, I found that to my left eye 
a distant lucid point had the appearance of a well defined line, 
corresponding exactly in direction, and nearly in length to the 
major axis of the ellipse above-mentioned. I found also that 
if I drew upon paper two black lines crossing each other at right 
angles, and placed the paper in a proper position, and at a 
certain distance from the eye, one line was seen perfectly distinct, 
while the other was barely visible: upen bringing the paper 
nearer to the eye, the line which was distinct now disappeared, 
and the other was seen very well defined. All these appearances 
indicated that the refraction of the eye was greater in the plane - 
nearly vertical, than in that at right angles to it, and that conse- 
quently it would not be possible to see distinctly by the assistance of 
lenses with spherical surfaces. I found, mdeed, that by turning 
a concave lens obliquely, or by looking directly through a part 
near the edge, I could see objects without confusion ; but in both 
cases, the distortion produced in their figure was such, that I 
could not hope to make any use of the left eye without some 
more effectual assistance. 
My object now was to form a lens which should refract more 
powerfully the rays in one certain plane, than those in the 
plane at right angles to it; and the first idea was to employ one 
whose surfaces should be cylindrical and concave, the axes of 
the cylinders crossing each other at right angles, and their radu 
being different. To shew that this construction would effect my 
purpose, it is only necessary to imagine the lens divided into two 
lenses by a plane perpendicular to its axis; then it is easily seen 
