Wireless Transmission of Photographs Part 8

If now the eye is placed six inches behind the screen and the screen removed, so that we can view the small image distinctly in the air, we shall see it with an apparent magnitude as much greater than if the same small image were equally far off with the tree, as 6 inches is to 5000 {136} feet, that is 10,000 times. Thus we see that although the image produced on the screen is 1000 times less than the tree from one cause, yet on account of it being brought near to the eye it is 10,000 times greater in apparent magnitude; therefore its apparent magnitude is increased as 10,000 1000, or 10 times. This means that by means of the lens it has actually been magnified 10 times. This magnifying power of a lens is always equal to the focal length divided by the distance at which we see small objects most distinctly, viz. 6 inches, and in the present instance is 60 6, or 10 times.

When the image is received upon a screen the apparatus is called a _camera obscura_, but when the eye is used and sees the inverted image in the air, then the apparatus is termed a _telescope_.

The image formed by a convex lens can be regarded as a new object, and if a second lens is placed behind it a second image will be formed in the same manner as if the first image were a real object. A succession of images can thus be formed by convex lenses, the last image being always treated as a fresh object, and being always an inverted image of the one before. From this it will be evident that additional magnifying power can be given to our telescope with one lens by bringing the image nearer the eye, and this is accomplished by placing a short focus lens between the image and the eye. By using a lens having a focal length of 1 inch, and such a lens will magnify 6 times, the total magnifying power of the two lenses will be 10 6 = 60 times, or 10 times by the first lens and 6 times by the second. Such an instrument is known as a _compound or astronomical telescope_, and the first lens is called the object gla.s.s and the second lens the magnifying gla.s.s, or eye-piece.

We are now in a position to understand how virtual images are formed, and the formation of a virtual image by means of a convex lens will be readily followed from a {137} study of Fig. 73. Let L represent a double convex lens, with an object, AB, placed between it and the point F, which is the princ.i.p.al focus of the lens. The rays from the object AB are refracted on pa.s.sing through the lens, and again refracted on leaving the lens, so that an image of the object is formed at the eye, N. As it is impossible for the eye to follow the bent rays from the object, a virtual image is formed and is seen at A^1B^1, and is really a continuation of the emergent rays. The magnifying power of such a lens may be found by dividing 6 inches by the focal length of the lens, 6 inches being the distance at which we see small objects most distinctly. A lens having a focal length of 1/4 inch would magnify 24 times, and one with a focal length of 1/100th of an inch 600 times, and so on. The magnifying power is greater as the lens is more convex and the object near to the princ.i.p.al focus. When a single lens is applied in this manner it is termed a _single microscope_, but when more than one lens is employed in order to increase the magnifying power, as in the telescope, then the apparatus is termed a _compound microscope_.

[Ill.u.s.tration: FIG. 73.]

Unlike a convex lens, which can form both real and virtual images, a concave lens can only produce a virtual image; and while the convex lens forms an image larger {138} than the object, the concave lens forms an image smaller than the object. Let L, Fig. 74, represent a double concave lens, and AB the object. The rays from AB on pa.s.sing through the lens are refracted, and they diverge in the direction RRRR, as if they proceeded from the point F, which is the princ.i.p.al focus of the lens, and the prolongations of these divergent rays produce a virtual image, erect and smaller than the object, at A^1B^1. The princ.i.p.al focal distance of concave lenses is found by exactly the same rule as that given for convex lenses.

[Ill.u.s.tration: FIG. 74.]

Up to the present we have a.s.sumed that all the rays of light pa.s.sed 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 princ.i.p.al 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 pa.s.s through the edges of the lens are brought to a focus nearer the lens than the rays that pa.s.s through the centre. Since this defect arises from the spherical form of the lens it is termed _spherical aberration_, and in lenses that {139} 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.

[Ill.u.s.tration: FIG. 75.]

[Ill.u.s.tration: FIG. 76.]

[Ill.u.s.tration: FIG. 77.]

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 circ.u.mference. 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 gla.s.s 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 b.i.+.c.hromate 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 glue, 120 Flexible couplings, 77 Frequency meter, 65 Friction brake, 88

{142} High speed telegraphy, 70 Hughes governor, 65 Hughes printing telegraph, 63 Hurter and Driffield, 104 Hydrogen, 100

Incidence, angle of, 127 Inertia, 64, 65, 111 effects in photo-telegraphy, 110 method of counteracting, 103, 112, 113 effect of wave-length of light on, 114 Intensifying solution, 122 Isochroniser, 89, 91 details of, 91, 92, 95 Isochronism, 64, 69, 70, 71

Kathode rays, 53 Knudsen, 2 apparatus of, 9 Korn, 30, 33, 45, 65, 72 apparatus of, 31

Lamps, coloured, 94 Lenses, 85, 125, 128 princ.i.p.al focus of, 130 conjugate foci of, 131 action of, 129 convex, 128, 131, 136 concave, 128, 138 focal length of, 130, 138 aperture, 138 meniscus, 139 Light, diffusion of, 86 extraneous, 87 Limit of error in synchronising, 64 Line balancer, 3 Line screens, 9, 15, 16, 116 making, 116

Magnifying power, 136, 137 Marconi valve, 44, 54 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 drying the, 121, 123 exposure of, 121 size of, 22, 24, 75, 77 pressing the, 22 Microscope, 131, 137 Military uses, 35 Mirror galvanometer, 9, 42, 73 Mirror, 47, 51 Morse code, 35 Motor speed, 89, 95 driving, 91, 93, 95 clockwork, 63 electric, 63

Nernst lamps, 43, 85, 98 heater of, 99 filament of, 99 principle of, 98 resistance of, 100 efficiency of, 101, 102 overrunning, 101 Nicol prism, 53

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 exposure of, 103, 107 speed of, 104, 105 plates, orthochromatic, 59 plates, 120 Points to be observed in preparing metal prints, 123 Poulsen Company, 32, 47 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

{143} Radio-photography, requirements of, 74 Refraction, angle of, 127 Refractive power, 127 Relay, 25, 39, 49, 53, 55, 60, 75 differential, 79 polarised, 97 working speed of, 26, 75 Reproducing for newspapers, 60 Resistance of selenium, 109 of selenium cells, 110 regulating, 113 r.e.t.a.r.dation of current, 6 Retouching, 62 Rotary spark-gap, 28

Selenium, 99 cells, 8, 34, 55, 60, 64, 109, 110 machines, 45 Self-induction, 24, 78 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 Stylus, angle of, 24, 78 defects of, 57 Submarine cable, 4 Synchronism, 11, 20, 36, 64, 69, 71

Telephograph, 74 advantages of, 76 method of working, 96 Telephone receiver, 83, 85 diaphragm, 48 improved, 51 Telephone relay, 48, 50, 52, 83, 85, 97 Telescope, 131, 136 Thermodetector, 32 Tow, 88 Transmission, distance of, 35, 72 speed of, 25, 35, 75

Vibration, natural period of, 39

Watkins, 105 power number, 105 Waves, damped, 30 undamped, 30, 31 Wheatstone bridge, 113 Wireless apparatus, 13 _Wireless World_, 31 Wynne, 105

Zirconia, 99

THE END

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