ON ASTRONOMY. 131 



directly behind the wheel, with a rectangular opening in it. The 

 form of the opening in the diaphragm and in the teeth of the wheel are 

 seen enlarged in a part of the wheel c' , figure 18. As the wheel c 

 revolves, the opening in the diaphragm is successively opened and 

 closed with great rapidity. If it were to be permanently open, then 

 the light from the lamp would fall upon the plane glass m, a portion 

 would be transmitted and a portion reflected, which latter portion 

 would pass through the lenses x and y, be reflected at n, return 

 again, fall a second time upon m, and be transmitted to the eye at e. 

 In this state of the apparatus the eye sees a bright point at the focus o. 

 The bright point continues visible until the velocity of the wheel c is 

 such that one of its teeth passes over the notch in the diaphragm and 

 thus prevents the reflected light from coming to the eye. The bright 

 point is then eclipsed. This will necessarily take place whenever the 

 time occupied by the tooth in passing the space of the notch is the 

 same as that required by light in going from o to n and returning. 



When the velocity of the wheel becomes so great as to close and 

 open the notch again while the light is going and coming, the bright 

 point will reappear. As the rotary speed of the wheel increases, the 

 point will be successively eclipsed. The rotary velocity of the wheel 

 is registered by appropriate mechanism so as to mark with precision 

 the number of turns to a second, while the number of teeth in the 

 wheel will serve to determine the time in which one tooth passes the 

 notch. The distance between the stations was 8,633 metres, equal 

 to five and one-eighth miles nearly. The apparatus indicated the 

 time of traversing the double distance, or ten and two-third miles, to 

 be about y-^o o" ^^ riro o^u <^f ^ second, which makes the velocity of light 

 very nearly the same as that given by the preceding methods. Physi- 

 cal science presents few more striking results than this of M. Fizeau. 

 From the near agreement of these three independent methods of 

 determining the velocity of light we may safely conclude that the 

 probable error is small. Light moves with the amazing velocity of 

 192,000 miles per second. 



We shall now proceed to inquire into the distance of the fixed stars. 

 And we may state, in the outset, that the only means of arriving at 

 an indication of this is an annual change in the position of the star. 

 But there are certain changes which are dependent upon known causes 

 which must first be allowed for. These are refraction, aberration, 

 and nutation. The refraction varies with the altitude of the star ; 

 aberration for any particular star depends on the season of the year ; 

 nutation, (another fine discovery of Dr. Bradley, in consequence of 

 which the star describes a little ellipse about its mean place once in 

 about nineteen years,) depends upon the position of the nodes of the 

 moon's orbit. This inequality is due to the action of the moon upon 

 the spheroidal figure of the earth. The amount of these corrections 

 is well known. Now, if there is any change in the position of a star 

 over and above what is duo to the above causes it must be referred 

 either to its own proper motion or to the change of position of the 

 earth in its orbit. But these two are not liable to be confounded the 

 one with each other. The proper motion will be progressive from year 

 to year in the same direction. Any change due to the motion of the 



