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Page:Popular Science Monthly Volume 4.djvu/685

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IMAGES AND SHADOWS.

IMAGES AND SHADOWS.
By W. J. YOUMANS, M. D.

SO much has been said lately of the wonders of spectrum analysis, that we are very apt to forget the other and equally marvelous properties of the agent by which it is produced. Spectrum analysis is a rare and curious experiment, but the more familiar effects of light which we daily experience are really just as wonderful, if we will but pause to reflect upon them. Science means knowledge, and the science of optics embodies our knowledge of light; but how much, after all, do we know of it? A great deal, undoubtedly, of its modes of action, but very little, if any thing, of its nature. We have an hypothesis or supposition about it, and work out ingenious conclusions, logically and experimentally, and say that they are proved; but how far are we from comprehending them.

That light moves at an amazing velocity, is shown in several ways; and all the methods bring us to about the same results which are expressed in numbers and are demonstrably true; but what finite mind can enter into the meaning of the statement that the luminous ray moves forward at the rate of 185,000 miles in a second of time? Between the two ticks of a second's pendulum, we are told that light

Fig. 1.
PSM V04 D685 Rectilinear propagation of light.jpg
Rectilinear Propagation of Light.

would pass round our globe seven and a half times. But who has a notion even of the dimensions of our globe? The number of thousands of miles through it and around it have been calculated, and the calculations harmonize with the whole body of astronomical knowledge; but we can form no adequate conception of such magnitudes. We patch together different shreds of our mental experience of large