THL PRESIDENTS ADDRESS. 



145 



from the outside points of the object will not focus on the same 

 point ; for ray I may come to a focus, say, at F 1? whilst ray II 

 is at F 2 . 



In Fig. 6 a, the sine-law being supposed to have been fulfilled, 

 it is quite evident that the two zones are yielding exactly the 

 same magnification, for they are seen to be both focussing at 

 the same point, F x ; whilst in the next illustration (Fig. 6 b), the 



IF SIME-LAW BE FUUHUED YI^AX'AL PO/»rS,\T CAVSESGCOD IMAGE OF OBJECT LY1 NG OurSWE X*-'>3 

 BECAUSE ZONES FOCUSAT S^/tE ro/A/r/T' 



Fig. GA. 



computer having neglected the sine-condition, the zones are shown 

 coming to different foci — one at F± and the other at F 2 . It is 

 obvious how disastrous such a state of things would be for the 

 definition ; and if bad for two zones onlv, I leave vou to imagine 

 what it would be if all the zones of the object-glass were being 

 considered. We should have a line extending from F 1 to F 2 

 instead of two points. But even yet I have not quite finished. This 

 class of defect does not, I believe, ever occur quite alone ; other 

 evil genii associate themselves, rendering " confusion worse con- 

 founded," their evil influences making their presence apparent 



f-r/T 



RESULTOF -WA'-FULFILMENT 



Fig. 6 b. 



by causing the line, of which I have just been speaking, to be 

 spread out more and more into something like a comet's tail. 



Now, gentlemen, we have arrived at last at the end of the 

 argument, for this comet-like smudge indicates what is called 

 coma. I think from what I have said you will all understand 

 how important a thing it is with objectives for the microscope 

 that this law should always be faithfully carried out, and why 



Jourx. Q M. C, Series II.— No. 56. 10 



