The completely equipped telescope, with its object-glass and mounting, aside from being a triumph of the highest optical and mechanical skill, is certainly the noblest instrument that man has yet constructed, and it is difficult to decide which is the most sublime and elevating to contemplate—the universe, which the telescope enables us to see, measure, weigh, and, combined with the spectroscope, to analyze; or the exquisite mechanism, by means of which light is first originated, then propagated, and finally refracted to an image on the retina of the eye.
We shall, in what follows, briefly consider the latter subject, which will enable lis to understand the natural laws that render possible the remarkable degree of perfection and power to which the refracting telescope has been carried, and which also fix a limit to its indefinite improvement.
Light is the sensation produced on the retina of the eye by some force, usually emanating from a luminous body, but not always, for the same sensation may also be produced by a current of electricity, or by a quick blow on the ball of the eye.
At the first glance this force, which has such a remarkable effect upon the retina of the eye, seems to be a rather difficult thing to interrogate in a way to make it divulge something of its true nature; and so it really proved, for even Sir Isaac Newton, with all the facts known in bis day, and with the splendid work of Huygens on the undulatory theory of light before him, failed to satisfy himself on that point; and, in fact, it required the combined work of Young, Fresnel, and many others, extending over a period of two hundred years, to demonstrate beyond question that the one and only explanation admissible is the undulatory theory first propounded by Huygens.
At the present time, however, it is possible to state with certainty a great deal regarding the true character of this force called light.
A revolving mirror, properly combined with one that is stationary, shows that light travels between them through a vacuum with the almost inconceivable velocity of 186,000 miles per second; while other experiments prove that this is also the velocity of light through space from star to star.
The diverse and curious phenomena called diffraction, interference, and dispersion, show that light consists of vibrations or waves in some transmitting medium, and therefore that this medium must fill the whole visible universe.
The phenomenon called polarization of light shows that the motion of each particle of the medium as it vibrates is at right angles to the direction in which the waves are propagated, and, strange to say, that the medium transmitting them has the properties of a solid substance, and not those of a fluid, such as a liquid or a gas. A good idea of this kind of a wave is had by observing the wave propagated along a tightly stretched telegraph-wire when it is struck a smart blow with a
