What a marvellous sense then is Sight, when we find that, not only can it grasp these innumerable vibrations, but can actually differentiate colours, appreciating as a different colour each increase of about one-tenth in these multitudinous frequencies; and it is principally by means of this Sense of Sight that we gain a knowledge of what is happening around us. And yet what strides we have made in the last two hundred years to improve upon that instrument! With all its wonderful capabilities, we shall see later on that the eye is a very imperfect instrument for seeing very small objects, or even large objects when at a great distance. With the present compound Microscope, only developed in the last hundred years, and its apochromatic lenses, invented only in the last forty years, we are able to see and photograph objects of a minuteness immeasurably beyond the power of the human eye, and, with our telescopes, we can see and photograph stars far beyond the possibility of vision by the unaided eye; and yet, by the stellar spectroscope, we are actually able to examine and identify the very atoms of which that distant star is composed, or rather was composed hundreds of thousands of years ago; we can compare those atoms with the same atoms in our laboratories, and we find that, though the former are hundreds of thousands of years older than the latter, they show absolutely no signs of wear or loss of energy, though they have been for that enormous time, and are still, pulsating at the rate of not only millions but billions of times per second; and though the pulsations they emit have travelled across such a vast depth of space that the mind cannot even imagine the distance, there has not been any diminution in the numbers of pulsations per second, nor the slightest slowing down of the rate of flight at which they started on their journey from that far-off world. If there had been the _slightest_ change we could detect it at once by means of the Spectroscope.