It must first be pointed out that there is a limit to the power of the ordinary microscope, beyond which the skill of the optician cannot go. He is baffled at that point not because of any lack of ability on his own part, but because of the nature of light itself. An opaque object, unless it be self-luminous, which few things are, can only be seen by reflected light. Generally speaking, we see things because they reflect in some degree the light which falls upon them. But light consists of waves, and when we reach an object so minute that its diameter is about half the wave-length of light, then we cannot see it because it is unable to reflect the light on account of its smallness. We can see this any day by the seaside, or by a river or large pond. There it is evident that the waves and ripples are reflected by such things as large stones, wood posts or anything of any size which come in their way; but when a wave encounters an object much smaller than itself it simply swallows it up, as it were, flows all over it or around it, without being in any way reflected by it. And it is just the same with the waves of light; they are unaffected by obstacles below a certain size, and so are not reflected by them. For this reason things smaller than about a seven-thousandth of a millimetre cannot possibly be seen by a microscope in the ordinary way. But if an object can be made self-luminous, then it can be seen, whatever its size, if the magnifying power of the microscope be great enough. So this ultra-microscope, as it is called, is really an ordinary microscope of the highest power possible, with an added apparatus for making the tiny particles which are being sought for self-luminous. This is done by directing upon them a pencil of light of exceeding intensity. Generated by powerful arc lamps, the light is concentrated by a system of lenses until it is of an almost incredible brightness, after which it falls upon the object. Now at first sight this seems to be no different from the usual procedure with a microscope, and there appears to be no reason why it should be more successful, but the explanation is this: light is a form of energy, and the waves of this very intense beam, falling upon the object, throw it into a state of violent agitation, by virtue of which it shines, not with reflected light, but with light of its own. It is not that the waves are reflected, but that they so shake up the particle that it gives off light waves itself. And thus it comes within the range of human vision.