no-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.

* * * * *

{141}

INDEX

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

De' Arsonval galvanometer, 47, 73
Decoherer, 41
Design of machines, 21
Detectors, 83
Developing solutions, 105, 122
Diaphragm, movement of, 48, 52, 84, 87
Dipping rods, 81, 83
Distance of transmission, 33
Duration of wave-trains, 22, 25

Early experiments, 2
Einthoven galvanometer, 32, 44, 45, 54, 113
Electric clock, 93
Electrolytic receiver, 4, 37, 54, 61, 64
Enlarging arrangements, 124, 125
Experimental machine, 20
Extraneous light, 47

Fastening electrolytic paper, 58
Fatigue of selenium cell, 64, 114
Fish g