LIGHT. 



cular. It is perhaps not an unfair statement of the actual 

 condition of these two celebrated hypotheses, to say that while 

 the corpuscular system is found sufficient to explain most of the 

 common and obvious phenomena of optics, it totally fails in 

 explaining many of the most remarkable effects brought to light- 

 by modern observations and experiments. On the other hand, 

 the undulatory theory in general offers a satisfactory expla- 

 nation for all. This circumstance has very properly and legiti- 

 mately enlisted under that hypothesis almost all the leading 

 scientific men of the present day. 



Although the principal facts which we shall have now to 

 explain are in fact independent of either of these two hypotheses, 

 and incontestably true, whichever may be adopted, yet in their 

 exposition it will be necessary to adopt the language of one or 

 the other of these theories. We shall, for the reason just stated, 

 use the nomenclature of the undulatory theory. 



"We are then to imagine light to consist of undulations propa- 

 gated through the universal ether, in the same manner as the 

 waves or undulations of sound are propagated through the air. 



6. The first question then that arises is, what is the velocity 

 with which these waves move ? At what rate does light come 

 from a distant star to the eye ? Is it propagated instantaneously 1 

 Would a fire suddenly lighted at a point one hundred millions of 

 miles from the eye be seen at the moment the light was pro- 

 duced 1 or would an interval of time be necessary to allow the 

 light to reach the eye ? and if so, what would be the interval of 

 time in relation to the distance of the luminous object ? 



In tracing the progress of human knowledge, we frequently 

 have occasion to behold with surprise, and not without a due 

 sense of humility, the important part which accident plays in 

 the advancement of science. Often are we with diligent zeal in 

 search of things, which, if found, would be of trifling or no 

 value, when we stumble on inestimable treasures of truth. The 

 frequency of this strongly impresses the mind with the persuasion 

 that there is in secret operation a power whose will it is that 

 knowledge and the human mind should be constantly progressive. 

 It is in physics as in morals. We ignorantly seek that which is 

 worthless, and find what is inestimable. 



In the pursuit of knowledge we might well say that which we 



are taught to express in the pursuit of what is moral and good. 



We might say that the power which governs its progress knows 



better than we do "our necessities before we ask, and our 



ignorance in asking." We shall see a striking example of this in 



the narrative of the celebrated discovery of the motion of light. 



Soon after the invention of the telescope, and the consequent 



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