The camera sees things invisible to the human eye. Its most effective work is done with beams which are beyond human perception. The photographer uses the Actinic rays. Ordinary light is composed of the seven primary colors, of which the lowest in the scale is the red, and the highest to violet.

Those below the red are called the Infra-red, and they are the Hertzian waves, or those used in wireless telegraphy. Those above the violet are called Ultra-violet, and these are employed for X-ray work. The former are produced by the high tension electric apparatus, which we have described in the chapter relating to wireless telegraphy; and the latter, called also the Roentgen rays, are generated by the Crookes' Tube.

This is a tube from which all the atmosphere has been extracted so that it is a practical vacuum. Within this are placed electrodes so as to divert the action of the electrical discharge in a particular direction, and this light, when discharged, is of such a peculiar character that its discovery made a sensation in the scientific world

The reason for this great wonder was not in the fact that it projected a light, but because of its character. Ordinary light, as we see it with the eye, is capable of being reflected, as when we look into a mirror at an angle. The X-ray will not reflect, but instead, pass directly through the glass.

Then, ordinary light is capable of refraction. This is shown by a ray of light bending as it passes through a glass of water, which is noticed when the light is at an angle to the surface.

The X-ray will pass through the water without being changed from a straight line. The foregoing being the case, it was but a simple step to conclude that if it were possible to find a means whereby the human eye could see within the ultra-violet beam, it would be possible to see through opaque substances.

From the discovery so important and far reaching it was not long until it was found that if the ultra-violet rays, thus propagated, were transmitted through certain substances, their rates of vibration would be brought down to the speeds which send forth the visible rays, and now the eye is able to see, in a measure at least, what the actinic rays show.

This discovery was but the forerunner of a still more important development, namely, the discovery of radium. The actual finding of the metal was preceded by the knowledge that certain minerals, and water, as well, possessed the property of radio-activity.

Radio-activity is a word used to express that quality in metals or other material by means of which obscure rays are emitted, that have the capacity of discharging electrified bodies, and the power to ionize gases, as well as to actually affect photograph plates.

Certain metals had this property to a remarkable degree, particularly uranium, thorium, polonium, actinium, and others, and in 1898 the Curies, husband and wife, French chemists, isolated an element, very ductile in its character, which was a white metal, and had a most brilliant luster.

Pitchblende, the base metal from which this was extracted, was discovered to be highly radio-active, and on making tests of the product taken from it, they were surprised to find that it emitted a form of energy that far exceeded in calculations any computations made on the basis of radio-activity in the metals hitherto examined.

But this was not the most remarkable part of the developments. The energy, whatever it was, had the power to change many other substances if brought into close proximity. It darkens the color of diamonds, quartz, mica, and glass. It changes some of the latter in color, some kinds being turned to brown and others into violet or purple tinges.

Radium has the capacity to redden the skin, and affect the flesh of persons, even at some considerable distance, and it is a most powerful germicide, destroying bacteria, and has been found also to produce some remarkable cures in diseases of a cancerous nature.

The remarkable similarity of the rays propagated by this substance, with the X-rays, lead many to believe that they are electrical in their character, and the whole scientific world is now striving to use this substance, as well as the more familiar light waves of the Roentgen tube, in the healing of diseases.

It is not at all remarkable that this use of it should first be considered, as it has been the history of the electrical developments, from the earliest times, that each successive stage should find advocates who would urge its virtues to heal the sick.

It was so when the dynamo was invented, when the high tension current was produced; and electrical therapeutics became a leading theme when transmission by induction became recognized as a scientific fact.

It is not many years since the X-rays were discovered, and the first announcement was concerning its wonderful healing powers.

This was particularly true in the case of radium, but for some reason, after the first tests, all experimenters were thwarted in their theories, because the science, like all others, required infinite patience and experience. It was discovered, in the case of the X-ray, that it must be used in a modified form, and accordingly, various modifications of the waves were introduced, called the m and the n rays, as well as many others, each having some peculiar qualification.

In time, no doubt, the investigators will find the right quality for each disease, and learn how to apply it. Thus, electricity, that most alluring thing which, in itself, cannot be seen, and is of such a character that it cannot even be defined in terms which will suit the exact scientific mind, is daily bringing new wonders for our investigation and use.

It is, indeed, a study which is so broad that it has no limitations, and a field which never will be exhausted.