Up to the present we have assumed that all the rays of light passed through a convex lens were brought to a focus at a point common to all the rays, but this is really only the case with a lens whose aperture does not exceed 12°. By aperture is meant the angle obtained by joining the edges of a lens with the principal focus. With lenses having a larger aperture the amount of refraction is greater at the edges than at the centre, and consequently the rays that pass through the edges of the lens are brought to a focus nearer the lens than the rays that pass through the centre. Since this defect arises from the spherical form of the lens it is termed spherical aberration, and in lenses that are used for photographic purposes the aberration has to be very carefully corrected.
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 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 167/100ths of its thickness. In a plano-convex lens with the plane side turned towards parallel rays the aberration is 41/2 times its thickness, but with the convex side turned towards parallel rays the aberration is only 117/100ths 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, 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.
Aberration, 139
Acetylene gas lamps, 120
Actinic power, 102
Actinograph, 105
Actinometer, 120
Ammonia, 123
Aniline dye, 123
Ballasting resistance, 100
Belin, 47
Berzelius, 109
Bichromate of potash, 120
Blondel's oscillograph, 47
Camera obscura, 136
choice of, 117
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
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
Distance of transmission, 33
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 glue, 120
Flexible couplings, 77
Frequency meter, 65
Friction brake, 88
High speed telegraphy, 70
Hughes governor, 65
Hughes printing telegraph, 63
Hurter and Driffield, 104
Hydrogen, 100
Incidence, angle of, 127
effects in photo-telegraphy, 110
method of counteracting, 103, 112, 113
effect of wave-length of light on, 114
Intensifying solution, 122
Lamps, coloured, 94
principal focus of, 130
conjugate foci of, 131
action of, 129
aperture, 138
meniscus, 139
Light, diffusion of, 86
extraneous, 87
Limit of error in synchronising, 64
Line balancer, 3
making, 116
coherer, 40
Mechanical inertia, 33
Mercury break, 81
churning of, 82
containers, 82
Mercury jet interrupter, 29
Metal prints, 15, 18, 32, 59, 64, 95, 120, 124
exposure of, 121
pressing the, 22
Military uses, 35
Mirror galvanometer, 9, 42, 73
Morse code, 35
clockwork, 63
electric, 63
Paper for electrolytic receiver, 56
Parabolic reflector, 8
Period of galvanometer, 43, 44, 46
Photographic Daily Companion, 105
Photographic films, 40, 43, 45, 53, 54, 62, 85, 86, 98
process, 37
chemical inertia, 103
plates, orthochromatic, 59
plates, 120
Points to be observed in preparing metal prints, 123
arc, 31
Preparing selenium, 109
photographs for transmitting, 15, 115
sketches on metal foil, 124
Prism, 128
action of, 129
Process plates, 122
Professor Nernst, 98
Radio-photography, requirements of, 74
Refraction, angle of, 127
Refractive power, 127
Relay, 25, 39, 49, 53, 55, 60, 75
differential, 79
polarised, 97
Reproducing for newspapers, 60
Resistance of selenium, 109
of selenium cells, 110
regulating, 113
Retardation of current, 6
Retouching, 62
Rotary spark-gap, 28
Selenium, 99
cells, 8, 34, 55, 60, 64, 109, 110
machines, 45
Sensitiveness of selenium cells, 113
ratio of, 113
Silvered quartz threads, 44, 46
Spark-gap, 27
Speed regulator, 68
adjustments of, 69
Spring clutch, 71
Starting position of machines, 98
String galvanometer, 32
Stylus, 17, 18, 57, 61, 78, 95, 103
sparking at, 24
defects of, 57
Submarine cable, 4
Telephograph, 74
advantages of, 76
method of working, 96
diaphragm, 48
improved, 51
Telephone relay, 48, 50, 52, 83, 85, 97
Thermodetector, 32
Tow, 88
Vibration, natural period of, 39
Watkins, 105
power number, 105
Waves, damped, 30
Wheatstone bridge, 113
Wireless apparatus, 13
Wireless World, 31
Wynne, 105
Zirconia, 99
THE END
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