172 THE POPULAR SCIENCE MONTHLY. 



The completely equipped telescope, with its object-glass and mount- 

 ing, 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 us 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 in- 

 terrogate 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 his day, and with the splendid work of Huygens on the un- 

 dulatory 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 ad- 

 missible 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 



