130 



LECTUEES 



angle with the visual ray from the star. In the triangle E B S' the 

 angle S' E B, very nearly equal to S E B, is always known from the 

 position of the star, the angle E S' B r=: S E S' is known hy ohser- 

 vation, E B = velocity of the earth, B S' = E S = velocity of light; 

 we then have sin. E S' B : sin. S' E B : : E B : B S'. But twice 

 in the year E S' B = 20" and S' E B = 90°, we then have- 

 sin. 20" : R : : vel. of earth : vel. of light ; 

 or, converting the R = sin. 90° in seconds and leaving off the sines, 

 we have — 



20" : 206265" : : vel. of earth : vel. of light. 



-TTi 11 1 -i i? T 1 J. 206265 X vel. of earth. 



Whence the velocity oi light =: ^ 



If we introduce into this equation the velocity of the earth, which 

 is nearly nineteen miles per second, we shall obtain a result very 

 nearly equal to 192,000, as given by the eclipses of Jupiter's satellites. 



The effect of aberration for the entire year will be seen by figure 

 17, where E E' E", &c., is the earth's orbit, S the mean place of a 

 star, and S' S" S"', &c., the apparent orbit described by the star, or 

 the ellipse of aberration. 



There is a third determination derived from direct experiment, 

 which is too important to be omitted in this connexion. In 1849, M. 

 Fizeau, of Paris, constructed a most ingenious apparatus, which had 

 the requisite delicacy for so difficult an experiment. The outline of 

 it is given in figure 18. 



J y 18 



-liL 



s is a lamp, u a lens placed in a tube which serves to converge a 

 beam of light on m, a plane unsilvered glass, placed in a tube ; thei 

 glass m reflects the convergent beam of light to a focus at o. x is 

 lens which receives the light and refracts it into a parrallel beam x y; 

 at y is another lens, which converges it to a focus on the plane mirror w, 

 from which it is reflected directly back, passes again through the lens 

 y, thence through the lens x, thence through the unsilvered glass m, 

 and reaches the eye at e. At the point o, where the light is brought 

 to a focus, an opening is made in the tube in which the toothed 

 wheel c is made to revolve with great velocity by mechanism not 

 here represented. There is also a diaphragm placed across the tube, 



