THE VELOCITY OF LIGHT. 217 



reflecting collimator. It consists of a telescope pointed 

 directly upon the observing telescope. The eye-piece is 

 removed and replaced by a reflector which slides into the 

 place of the eye-piece, and exactly reaches to the principal 

 focus of the telescope. We see now that the rays coming 

 parallel from the observer's telescope, fall upon the object- 

 glass of the reflecting collimator, and are brought to 

 a focus upon the small mirror. The reflected rays diverge 

 to the object glass, and if the instrument is in perfect 

 adjustment, they must retrace their path. Thus they fall 

 parallel upon the object-glass of the observer's telescope, 

 and are brought to a focus at exactly the same part of the 

 notched disc as they emerged from. Thence they fall upon 

 the plane glass, and while some of the rays are, of course, 

 lost by reflection, a considerable part are sent through, and 

 can be observed through the eye piece. 



We have now traced the rays of light from the source of 

 light to the distant reflector and back ; and I wish to draw 

 your attention specially to three results of this arrange- 

 ment, which are highly instructive to all those who are 

 engaged on optical experiments. 



1 . You notice that the rays that pass to the reflecting 

 collimator diverge from a definite part of the notched disc, 

 and after their return are brought back to exactly the same 

 point. 



2. The collimator does not require to be perfectly 

 adjusted a thing that would be impossible, and that 

 renders the employment of a plane mirror useless, for if it 

 be not directed perfectly, still the rays will be returned on 

 their own path, so long as the mirror is exactly in the focus 

 of the object-glass. 



3. There is very little loss of light if the instruments 

 are in perfect focus. This was important to Fizeau, as it 

 enabled him to use a long distance. 



You can see the dimensions of the apparatus by inspec- 

 tion. The object-glasses are about 2| inches diameter, and 

 30 inches focal length. The disc is 4 inches diameter, and 

 has 1,000 teeth. The distance employed by Fizeau was 

 more than 5 miles ; or exactly 8,633 metres. This was at 

 Paris, from the height of Montmartre to a house at 

 Suresnes. He found that the first eclipse occurred when a 

 rotation of 12*6 turns a second was given to the disc. The 



