By TEMPEST ANDERSON, M.D., B. Sc.
The author has had constructed a cylindrical lens in which the axis remains
constant in direction and amount of refraction, while the refraction in the
meridian at right angles to this varies continuously.
A cone may be regarded as a succession of cylinders of different diameters
graduating into one another by exceedingly small steps, so that if a short
enough portion be considered, its curvature at any point may be regarded as
cylindrical. A lens with one side plane and the other ground on a conical
tool is therefore a concave cylindrical lens varying in concavity at
different parts according to the diameter of the cone at the corresponding
part. Two such lenses mounted with axes parallel and with curvatures
varying in opposite directions produce a compound cylindrical lens, whose
refraction in the direction of the axes is zero, and whose refraction in
the meridian at right angles to this is at any point the sum of the
refractions of the two lenses. This sum is nearly constant for a
considerable distance along the axis so long as the same position of the
lenses is maintained. If the lenses be slid one over the other in the
direction of their axes, this sum changes, and we have a varying
cylindrical lens. The lens is graduated by marking on the frame the
relative position of the lenses when cylindrical lenses of known power are
neutralized.
Lenses were exhibited to the Royal Society, London, varying from to -6 DCy,
and from to +6 DCy.
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