By making use of two plano-convex lenses placed together as at Fig. 76, the aberration will be one-fourth of that of a single lens, but the focal length of the lens, L1, must be half as much again as that of L. If their focal lengths are equal the aberration will only be a little more than half reduced. Spherical aberration, however, may be entirely destroyed by combining a meniscus and double convex lens, as shown in Fig. 77, the convex side being turned to the eye when used as a lens, and to parallel rays when used as a burning glass or condenser.
INDEX
Aberration, [139]
Accuracy of working, [70], [72]
Acetylene gas lamps, [120]
Actinic power, [102]
Actinograph, [105]