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letters more than the body of the plate. Adjoining the objects mentioned may be seen a pair of insulated wires tightly twisted together. In the ray-print the insulation is almost invisible, and the wires stand wide apart. The three irregular pieces seen near the twisted wire are coal: one piece, in the form of a wedge or prism, is of anthracite three eighths of an inch at its thickest part, and tapering to an edge; the other two are of bituminous coal varying in thickness from one sixteenth of an inch to over one fourth of an inch. The coal is relatively quite transparent, and the bituminous somewhat more so than the anthracite. The negative clearly shows by darker marks the presence of seams probably richer in earthy matter.

In this connection I make a suggestion. Instead of analyzing the coal for ash percentage, take a cathodograph of a definite thickness of it along with coals bearing known. percentages of ash, and compare the shadows. At the upper left-hand corner of the photograph on this page is a small hardwood awlhandle, the awl being broken. The cathodograph shows the brass ferrule to be opaque, and shows that the wood is fairly transparent-sufficiently so to reveal the end of the broken awl above the ferrule in the wood.

Adjoining the lettered aluminium plate was placed a piece of cellulose about one thirty-second of an inch thick. It was too transparent to show in the cathodograph.

By the cellulose was a piece of bromide print in black, with white letters, «Assembly.» This was too transparent to be visible in the shadow-picture. The keyhole escutcheon seen near it is of iron, about one sixteenth of an inch thick, with chamfered edges. It is opaque, but by inspection of the negative the edges are clearly seen to transmit rays. In the upper right-hand corner of the photograph is a cork about one inch in diameter, having two glass tubes passing through it. The cork has disappeared in the shadow-picture, being too transparent. (Cathodography of corked bottles would uncork them.) The glass tubes are seen to be partly transparent.

It is indeed surprising to find the dense black coal masses transmitting the rays so freely that an inch or more in thickness would be no particular obstacle to the taking of a picture.

In developing a cathodograph picture it is noticeable that the development goes on all through the film, back as well as front. This is not the case with ordinary camera exposures, which develop from the front toward the back of the plate. The behavior noted is with the cathodograph an indication of the fact that the sensitive film itself is largely transparent to the cathode rays, and therefore lets them through without fully utilizing them. If they could all be absorbed and made to do chemical work, our time for making an impression would be much abbreviated.

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To test this, I placed in front of a sensitive plate, and between it and its slide in the plateholder, four thicknesses of sensitive bromide paper. It was then placed under the Crookes tube, with a few objects-one of which was a small permanent magnet-on the cover of the plate-holder containing the plate and paper. There were thus five superposed sensitive layers traversed by the rays. A picture was obtained on each of them. These pictures were of equal intensity, and the glass had a strong impression. The indication that a dozen or twenty paper pictures might have been obtained at one time, I have since verified.

This looks as if only a small portion of the silver bromide is susceptible to these rays, and that to get the best results special preparation of the sensitive materials will be needed; or the impression may depend on fluorescence, in which case strongly fluorescent chemically inert powders should be incorporated with the sensitive substances.

MADE BY ELIHU THOMSON.

There is every reason to believe that much greater speed will be attained by the use of highly sensitive materials and of greater energy in the Crookes tube apparatus.

The detection of cathode rays by fluorescence will undoubtedly be of great service in saving time in making explorations in surgery,

etc. With a highly fluorescent screen placed within the range of vision, in a dark box provided with sight-holes, the observer will be able at once to detect the presence or absence of the rays, the forms of the shadows, etc., instead of photographing them. He will be able also easily to change the direction of the rays and make observations in the different directions without loss of time.

It is possible also that an exploring apparatus, consisting of a negatively electrified body with an electroscope, may be so arranged as to give a record, or map, as it were, of the shadows cast by the cathode rays. In this case the sensitiveness can be made exceedingly great.

It is too early to settle upon any theory as to the nature of the rays. They agree in several particulars with what is called ultraviolet light, or with ultra-violet rays. Yet they are not refracted or reflected, or at least no observations have as yet been made showing that they possess the capability of refraction or reflection. Certain delicate markings which I have observed on a few cathodographs would almost indicate a trace of refraction or reflection existing; still, there may be another explanation of these markings.

May it not be that high-pitch waves in the ether, even when transverse like light-waves,

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HAND OF A GIRL, AGED 11, SHOWING DISEASED BONES. CATHODOGRAPH MADE BY PROFESSOR S. P. THOMPSON.

can pass between the molecules and travel in free ether between them, or otherwise undergo absorption in the molecule itself when the latter is across the path?

Be they what they may, the study of cathode rays will open up the way to further discoveries in that borderland between matter and ether. We know that magnetism concerns the ether far more than it does ordinary matter. We know that light and radiant heat are electromagnetic vibrations of high pitch in the ether. It is more than probable that gravitation is dependent on some form of ether vibration. We shall await the proof of the true nature of cathode rays, fully assured that it will come in due time.

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FOR success in producing electric shadows the first requisite is a Crookes tube having a very perfect vacuum. The highest exhaustion is essential, many tubes failing from having only a sufficient vacuum to produce phosphorescence. The glass should be thin and preferably containing no lead. An ordinary Ruhmkorff coil, capable of throwing a spark four or five inches in length, suffices to stimulate the tube, provided its break works with sufficient rapidity. With the ordinary slow break very long exposures are necessary. If the bulb or tube is large, sharply defined shadows will not be obtained. The rays that produce the shadow effects do not emanate exclusively from the phosphorescent patches on the glass bulb, as Roentgen himself sup

posed. One of my tubes cast distinct double shadows, the stronger shadow falling in a direction as if the rays had come straight from the cathode, the fainter shadow in a direction straight from the phosphorescent patch on the glass. Interposing a piece of pine-wood, I have found shadows of the grain of the wood upon the sensitive plate, which proved that the dark resinous streaks are more transparent to these rays than the lighter-colored tissue. I have obtained shadows of coins shut up in a leather purse, of pens inclosed in a wooden box, of a pair of spectacles lying in its case, and of gems of various sorts inclosed in a wooden box. Olivine appears to be more opaque than topaz, sapphire, or diamond. Diamond is more opaque than black carbon of equal thickness. A piece of amber inclosing flies shows no shadows of the flies. I have found no difficulty in getting shadows of the bones of the hand, even down to the wrist; but in my sciographs, as in those of Mr. Swinton, the flesh always casts a shadow also. Magnetized iron and non-magnetized iron appear to be equally opaque. Bones are not very opaque: one notices in the shadows of the carpal bones of the hand that the enlarged ends, where there is marrow, are more transparent than the parts where the bone is denser. At present there is not the slightest evidence that these rays can be polarized; that is the strongest argument in favor of the view that they consist of longitudinal vibrations. Neither, as yet, has any means been found of reflecting them. They apparently agree with ultra-violet light in one respect only, that of frequency. But of their wave-length, whether it is shorter or longer than that of ultra-violet light, nothing is certain. It depends on the velocity of propagation of the rays as to whether it is less or greater. The name of «ultra-violet sound» appears to me to be appropriate to the phenomenon. The thing most wanted now is a more powerful means of exciting the rays so as to shorten the time of exposure.

I find that metallic sodium is more transparent than metallic potassium, and slightly more transparent than aluminium. Metallic lithium is far more transparent than either. It is certainly ten times more transparent than aluminium, and can hardly be caused to cast any shadow at all. In fact, it appears as though the opacity of bodies to the Roentgen rays was proportional, not to density, but to atomic weight. Imitation rubies made of red glass are more opaque than natural rubies; imitation pearls are more opaque than real pearls. To obtain good sciographs of hands

VOL. LII.-17.

and feet, showing the bones only, a longer exposure is needed than will suffice to show the bones through the flesh. With prolonged exposure the flesh disappears, the rays penetrating it more and more.

The negatives frequently show as though much more fully exposed on the side next the glass than on the front of the film. This gives color to the notion that the photographic effect is of a secondary order, the Roentgen rays penetrating the film to the surface of the glass, where by some transmutation akin to phosphorescence they generate the photographic effect. Abney says that if the sensitive films are spread on ferrotype iron instead of on glass no shadows are produced.

The statement attributed to me in various quarters, that I have found Roentgen rays in the arc-lamp, has never been made by me. What I have found is that, using an arc-lamp, I could get photographic shadows of metal objects through a wooden screen; but they are stopped by an aluminium sheet, and hence are not due to Roentgen rays.

I have succeeded in reading the contents of a sealed letter by the Roentgen method. The ink was of a metallic nature; writing in vegetable inks produces no appreciable shadow.

In the sciograph of the hand of a child aged eleven, in St. Bartholomew's Hospital, London, suffering from growth of bony tumors upon the bones of hands, feet, etc., the distortions of the joints so produced are very plainly shown.

Silvanus P. Thompson.

CITY OF LONDON AND GUILDS TECHNICAL
COLLEGE, LONDON, ENGLAND.

AT the University of Toronto, as elsewhere, the announcement by Professor Roentgen of his discovery of a new form of radiation excited the most intense interest. Together with Mr. C. H. C. Wright and Mr. Keele of the School of Practical Science, I arranged a series of experiments to verify, if possible, the results said to have been obtained. Although we were successful in establishing the extraordinary penetrating power of the rays and their action on a sensitized photographic plate, we found that long exposures were necessary in order to obtain sharp and clearly defined images. As this limited to a very great extent the applications of the «new photography,» we directed our efforts to reducing, if possible, the time of exposure, and this we succeeded in doing to a very marked degree.

On making a careful test of all the tubes in the physical laboratory, we found one which

gave a much stronger radiation than any of the others. This tube was pear-shaped, and as it had one electrode inserted in the smaller end and the other in the side, we were able, by making the former the negative terminal, to obtain a large glass surface exposed to the action of the cathode rays. This tube was employed in all our later experiments. Thinking that probably the action would vary with different sensitized films, we conducted tests to determine the relative sensitiveness to the rays of various types of plates; but we observed marked difference, and concluded that any reduction in the time of exposure must be otherwise obtained. Experiments were also made with prisms and lenses of wood, pitch, and other materials, but no indication of refraction at their surfaces could be discovered.

The only remaining method for the concentration of the rays seemed to be an application of the principle of reflection. In order to determine whether the rays could be reflected, a surface of clean mercury was prepared, and it was found that when the rays were directed towards this sensitized films protected from direct radiation were fogged by some action coming from the mercury. To test this apparent reflection still further, a sensitized film, protected by a plate-holder, was placed at a distance of about twenty centimeters below the Crookes tube. A thick plate of glass was then inserted midway between the tube and the film, parallel with the latter, with the intention of screening the plate in part from the action of the rays. The tube was then excited for some time, and on developing the film it was found that the rays evidently traveled in straight lines, since the part of the film protected by the glass plate was well defined and entirely unaffected by them. This experiment was repeated, the arrangement of apparatus being identical, with the sole exception that a glass bell-jar was placed over the whole. Development of the film in this case showed (1) no action on the film outside the jar; (2) no indication that the interposed glass plate acted as a screen; (3) the action much more intense than in the previous experiment, proving conclusively the reflection of the rays from the surface of the jar.

By the employment of this method we reduced on February 11 the time of exposure almost to instantaneousness. The picture of the pendant given on page 121 was taken with the bell-jar over the apparatus, and was obtained by an exposure of four and a half seconds, the object being a medal placed within a leather

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ALL about and within us exist rates of vibration known as forms of energy, some of them forced by man's ingenuity to record themselves by aid of mechanisms, others yet awaiting this sort of detection. Recording devices to reveal laws of light, heat, or chemical affinity are familiar, but no one has in a similar manner recorded thought or gravitation. Electricity yields its secrets slowly. While Faraday divined and Maxwell proved mathematically its existence outside of the conductor, Hertz actually detected its vibration and its wave-lengths by means of his oscillator. Lenard detected and recorded cathodic rays outside of a vacuum-tube, and now Roentgen comes forward to show by a device that the leather of a purse and the flesh of the hand may be penetrated by a radiation, leaving coins within, and bones otherwise invisible, pictured. At once man's curiosity in uncovering the otherwise unseen became highly excited.

Reference to one's own personal participation in the development of the Roentgen process even now seems like a reminiscence, so rapidly have events moved onward. The announcement of its possibility found me fully equipped with all needful electrical apparatus, except a Crookes tube, which could not be purchased anywhere. I therefore turned my attention at once to the static machine and ordinary vacuum-bulbs, and with these simple appliances have progressed to a point of picturing all sorts of metallic objects upon six-inch by eight-inch, or even larger plates, in from three to five minutes, and have obtained a perfect shadowgraph of a small piece of a needle behind a bone in the foot. The bones of the hand have already become an old story. I find my X rays are very intense, so much so that over-exposure easily penetrates some bones and thin plates of metal.

My first experiments demonstrated that shadowgraphs could be obtained simply by causing a powerful spark from the static machine to pass around, but not through the plate-holder. One of my first working vacuumbulbs was a radiometer such as is commonly sold by opticians, to which I attached external electrodes. Another form of vacuum-bulb had one external and one internal electrode. The illustration on page 125 represents an early effort with the radiometer bulb. I now use some bulbs a foot in diameter and having no enter

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