The distortion of an image formed by a convex lens is shown by the diagram, Fig. 75. If we receive the image upon a sheet of white cardboard placed at A, we shall find that while the outside edges will be clear and distinct, the inside will be blurred, the reverse being the case when the cardboard is moved to the point B.
Aberration is to a great extent minimised by giving to the lens a meniscus instead of a biconvex form, but as it is desirable to reduce the aberration to below once the { 140} thickness of the lens, and as this cannot be done by a single lens, we must have recourse to two lenses put together. The thickness of a lens is the difference between its thickness at the middle and at the circumference. In a double convex lens with equal convexities the aberration is 1-67/100ths of its thickness. In a plano-convex lens with the plane side turned towards parallel rays the aberration is 4-1/2 times its thickness, but with the convex side turned towards parallel rays the aberration is only 1-17/100ths of its thickness.
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, L^1, 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.
Aberration, 139 spherical, 138, 140 Accuracy of working, 70, 72 Acetylene gas lamps, 120 Actinic power, 102 Actinograph, 105 Actinometer, 120 Alternating current, 82, 100 Ammonia, 123 Angle of stylus, 24, 78 Aniline dye, 123 Arcing, 27, 82 Arc lamps, 15, 120, 121 Atmospherics, 61, 85
Ballasting resistance, 100 Belin, 47 Bernochi, 7, 112 system of, 7, 34 Berzelius, 109 Bichromate of potash, 120 Blondel's oscillograph, 47
Camera obscura, 136 extension, 116, 118 choice of, 117 Capacity of condenser, 24, 78 electrostatic, 3, 5 of cable, 3 of London-Paris telephone line, 3 Carbon bisulphide, 53 Charbonelle, 48 receiver of, 48 Chemical solution, 56 Circuit breaker, 76 Clutch, details of, 88, 89, 91 spring, 71 Coating the metal sheets, 120 Coherer, 11, 40 Collecting rings, 91 Commercial value of photo-telegraphy, 1 Compensating selenium cell, 112 Contact breaker, 37 Copying arrangements, 118, 125 Cross screen, 21