LIGHT. 



225 



developed, and the general result has been that the undulatory theory has pre- 

 vailed over the corpuscular. It is perhaps not an unfair statement of the ac- 

 tual condition of these two celebrated hypotheses, to say that while the cor- / 

 puscular system is found sufficient to explain most of the common and obvious I 

 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 explanation 

 for all. This circumstance has very properly and legitimately 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 



a- pendent of either of these two hypotheses, and incontestably true, which- 

 ever 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 

 sta-ted, use the nomenclature of the undulatory theory. 



We are then to imagine light to consist of undulations propagated through 

 the universal ether, in the same manner as the waves or undulations of sound 

 are propagated through the air. 



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 ? Would a fire suddenly lighted at a point 

 one hundred millions of miles from the eye be seen at the moment the light 

 was produced ? 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 occasions 

 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 often 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 what we do, " our neces- 

 sities before we ask, and our ignorance in asking." We shall see a striking 

 example of this in the narrative which I shall now offer of the celebrated dis- 

 covery of the motion of light. 



Soon after the invention of the telescope, and the consequent discovery of 

 Jupiter's satellites, Roemer, an eminent Danish astronomer, engaged in a series 

 of observations, the object of which was the discovery of the exact time of 

 the revolution of one of these bodies around Jupiter. The mode in which he 

 proposed to investigate this, was by observing the successive eclipses of the 

 satellite, and noticing the time between them. 



Let S represent the sun and ABCDEFGH the successive positions 

 of the earth. Let J be Jupiter projecting behind him his conical shadow, and 

 let M N 0. represent the orbit of one of his satellites. After each revolution 

 the satellite will enter the shadow at M, and emerge from it at N. 



Now if it were possible to observe accurately the moment at which the sat- 

 ellite would, after each revolution, either enter the shadow, or emerge from it, 

 the interval of time between these events would enable us to calculate exactly 

 the velocity and motion of the satellite. But by attentively watching the sat- 

 ellite we can note the time it enters the shadow, for at that moment it is de- 



