THE PRESIDENT S ADDRESS. 



23 



those embellishments and errors removed, it is a consistent 

 working theory, its limit of resolving power agrees very well 

 with results practically obtained, and it also affords valuable 

 information for checking the interpretation of periodic struc- 

 tures. 



Our next point is a digression, which I trust you will pardon, 

 for we must investigate the theory of telescopic vision before 

 we can proceed. 



Mr. Wright has given a very clear explanation of the theory 

 of telescopic vision in the " English 

 Mechanic."* Very briefly stated it is 

 this : Let A B W (Fig. 1) be a long isos- 

 celes triangle with a narrow base A W. 

 Let B represent the focus, and A W the 

 diameter of a telescope objective. 



Then, if light having travelled along 

 A B arrives at B in a certain phase, it 

 will also arrive there in the same phase 

 when it has come via W B, because W B 

 is equal to A B. Now let us take another 

 point D, at one side of and close to B, 

 and let us draw lines from D to A and 

 W, then it is clear that the triangle 

 ADW will not be isosceles, for one side 

 must be longer than the other, and the 

 greater the distance of D from B the 

 greater will be this inequality of the 

 sides A D, W D of the triangle ADW. 

 Let the point D be placed at such a dis- 

 tance from B that the difference in the 

 lengths of the two sides A D and W D 

 of the triangle ADW amounts to half a 

 wave-length, it is then obvious that light 

 arriving at D via A D will differ in 

 phase from that coming via W D by half a wave-length. In 

 other words, to use a familiar figure, at the point B the crests 

 of the waves will meet the crests, and the hollows will meet 

 the hollows, consequently there will be a reinforcement of wave 

 action, but at the point D the crests will meet the hollows, and 

 vice-versa, so that there the wave motion will be annihilated, 

 * "English Mechanic," Vol. \x., No. 1540, p. 125. 



