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different absorbability, and the investigations of Hertz and Lenard show that the cathode rays are similarly to be discriminated. While the "softest" tubes investigated generated rays much less subject to absorption than any cathode rays investigated by Lenard, yet there is no reason to doubt the possibility of X-rays of greater absorbability, and cathode rays of less. It therefore appears probable that in future investigations rays will be found bridging over the gap between X-rays and cathode rays, so far as their absorption is concerned. (2) We found in section 4 that the specific transmissibility of a body becomes less the thinner the plate passed through. Consequently, had we made use in our experiments of plates as thin as those employed by Lenard it would have been found that the X-rays were more nearly like those of Lenard in their absorbability.

10. Besides the fluorescent phenomena, there may be excited by X-rays photographic, electric, and other actions, and it is of interest to know how far these various manifestations vary in similar ratio when the source of the rays is altered. I must restrict myself to a comparison of the first two phenomena.

A hard and a soft tube were so adjusted as to give equally bright fluorescence as compared by means of the photometer described in section 2. Upon substituting a photographic plate in the place of the fluorescent screen it was found, on development, that the portion subject to the rays from the hard tube was blackened to a less degree than the other. The rays, though producing equal fluorescence, were thus for photographic purposes unequally active.

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The great sensitiveness of a photographic plate even for rays from tubes of medium hardness is illustrated by an experiment in which 96 films were superposed, placed at a distance of 25 centimeters from the discharge tube, and exposed five minutes with due precautions to protect the films from the radiations of the air. A photographic action was apparent on the last film, although the first was scarcely over-exposed.

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If the intensity of the radiations is augmented by increasing the strength of the primary current, the photographic action increases in the same measure as the intensity of the fluorescence. In this case, as in the case where the intensity of the radiation was increased by an alteration of the distance of the fluorescent screen, the brightness of the fluorescence is at least approximately proportional to the intensity of the radiation. This rule should not, however, be too generally applied.

II. In conclusion, mention should be made of the following particulars:

With a discharge tube of proper construction, and not too soft, the X-rays are chiefly generated in a spot of not more than I or 2 millimeters diameter where the cathode rays meet the platinum plate. This, however, is not the sole source. The whole plate and a part of the tube walls emit X-rays, though in less intensity. Cathode rays proceed in all directions, but their intensity is considerable only near the axis of the concave cathode mirror, and, consequently, the X-rays are strongly emitted only near the point where this axis meets the platinum plate. When the tube is very hard and the platinum thin, many rays proceed also from the rear surface of the platinum plate, but, as may be shown by the pinhole camera, chiefly from the spot lying on the axis of the mirror. *

I can confirm the observation of G. Brandes that the X-rays are able to produce a sensation of light upon the retina of the eye. In my record book appears a notice entered in the early part of November, 1895, to the effect that when in a darkened chamber, near a wooden door, I perceived a weak appearance of light when a Hittorf tube upon the other side of the door was put in operation. Since this appearance was only once observed, I regarded it as a subjective, and the reason that it was not then repeatedly observed lay in the fact that other tubes were substituted for the Hittorf tube which were less completely evacuated and not provided with platinum anodes. The Hittorf tube furnishes rays of slight absorbability on account of its high vacuum, and, at the same time, of great intensity on account of the employment of a platinum anode for the reception of the cathode rays.

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With the tubes now in use I can easily repeat the Brandes experi

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Since the beginning of my investigation of X-rays I have repeatedly endeavored to produce diffraction phenomena with them. I obtained at various times, when using narrow slits, appearances similar to diffraction effects, but when modifications were made in the conditions for the purpose of thoroughly proving the accuracy of this explanation of the phenomena it was found in each case that the appearances were produced in other ways than by diffraction. I know of no experiment which gives satisfactory evidence of the existence of diffraction with the X-rays.

W. H. PREECE

WILLIAM MARCONI was born near Bologna, Italy, in 1875. He became an electrical engineer and early gave his attention to the transmission of the Hertz electric waves without wires. The description. given below is by W. H. Preece, who invented a system which proved successful over short distances some years before Marconi.

WIRELESS TELEGRAPHY: THE PREECE AND MARCONI SYSTEMS

Science has conferred one great benefit on mankind. It has supplied us with a new sense. We can now see the invisible, hear the inaudible, and feel the intangible. We know that the universe is filled with a homogeneous continuous elastic medium which transmits heat, light, electricity and other forms of energy from one point of space to another without loss. The discovery of the real existence of this "ether" is one of the great scientific events of the Victorian era. Its character and mechanism are not yet known to us. All attempts to "invent" a perfect ether have proved beyond the mental powers of the highest intellects. We can only say with Lord Salisbury that the ether is the nominative case of the verb "to undulate." We must be content with a knowledge of the fact that it was created in the beginning for the transmission of energy in all its forms, that it transmits these energies in definite waves and with a known velocity, that it is perfect of its kind, but that it still remains as inscrutable as gravity or light itself.

Any disturbance of the ether must originate with some disturbance of matter. An explosion, cyclone, or vibratory motion may occur in the photosphere of the sun. A disturbance or wave is impressed on the ether. It is propagated in straight lines through space. It falls on Jupiter, Venus, the Earth, and every other planet met with in its course, and any machine, human or mechanical, capable of responding to its undulations indicates its presence. Thus the eye supplies the sensation

of light, the skin is sensitive to heat, the galvanometer indicates electricity, the magnetometer indicates disturbances in the earth's magnetic field. One of the greatest scientific achievements of our generation is the magnificent generalization of Clerk-Maxwell that all these disturbances are of precisely the same kind, and that they differ only in degree. Light is an electromagnetic phenomenon, and electricity in its progress through space follows the laws of optics. Hertz proved this experimentally, and few of us who heard it will forget the admirable lecture on "The Work of Hertz" given in this hall by Prof. Oliver Lodge three years ago.

By the kindness of Prof. Silvanus Thompson I am able to illustrate wave transmission by a very beautiful apparatus devised by him. At one end we have the transmitter or oscillator, which is a heavy suspended mass to which a blow or impulse is given, and which, in consequence, vibrates a given number of times per minute. At the other end is the receiver or resonator, timed to vibrate to the same period. Connecting the two together is a row of leaden balls suspended so that each ball gives a portion of its energy at each oscillation to the next in the series. Each ball vibrates at right angles to or athwart the line of propagation of the wave, and as they vibrate in different phases you will see that a wave is transmitted from the transmitter to the receiver. The receiver takes up these vibrations and responds in sympathy with the transmitter. Here we have a visible illustration of that which is absolutely invisible. The wave you see differs from a wave of light or of electricity only in its length or in its frequency. Electric waves vary from units per second in long submarine cables to millions per second when excited by Hertz's method. Light waves vary per second between 400 billions in the red to 800 billions in the violet, and electric waves differ from them in no other respect. They are reflected, refracted and polarized, they are subject to interference, and they move through the ether in straight lines with the same velocity, viz, 186,400 miles per second-a number easily recalled when we remember that it was in the year 1864 that Maxwell made his famous discovery of the identity of light and electric waves.

Electric waves, however, differ from light waves in this, that we have also to regard the direction at right angles to the line of propagation of the wave. The model gives an illustration of that which happens along a line of electric force; the other line of motion I speak of is a circle around the point of disturbance, and these lines are called lines.

of magnetic force. The animal eye is tuned to one series of wave; the "electric eye," as Lord Kelvin called Hertz's resonator, to another. If electric waves could be reduced in length to the forty-thousandth of an inch we should see them as colors.

One more definition, and our ground is cleared. When electricity is found stored up in a potential state in the molecules of a dielectric like air, glass, or gutta-percha the molecules are strained, it is called a charge, and it establishes in its neighborhood an electric field. When it is active, or in its kinetic state in a circuit, it is called a current. It is found in both states-kinetic and potential-when a current is maintained in a conductor. The surrounding neighborhood is then found in a state of stress, forming what is called a magnetic field.

In the first case the charges can be made to rise and fall, and to surge to and fro with rhythmic regularity, exciting electric waves along each line of electric force at very high frequencies, and in the second case the currents can rise or alternate in direction with the same regularity, but with very different frequencies, and originate electromagnetic waves whose wave fronts are propagated in the same direction.

The first is the method of Hertz, which has recently been turned to practical account by Mr. Marconi, and the second is the method which I have been applying, and which, for historical reasons, I will describe to you first.

In 1884 messages sent through insulated wires buried in iron pipes in the streets of London were read upon telephone circuits erected on poles above the house tops, 80 feet away. Ordinary telegraph circuits were found in 1885 to produce disturbances 2,000 feet away. Distinct speech by telephone was carried on through one-quarter of a mile, a distance that was increased to 14 miles at a later date. Careful experiments were made in 1886 and 1887 to prove that these effects were due to pure electromagnetic waves, and were entirely free from any earthconduction. In 1892 distinct messages were sent across a portion of the Bristol Channel, between Penarth and Flat Holm, a distance of 3.3 miles.

Early in 1895 the cable between Oban and the Isle of Mull broke down, and as no ship was available for repairing and restoring communication, communication was established by utilizing parallel wires on each side of the channel and transmitting signals across the space by these electromagnetic waves.

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