( 250 ) 

 III. — Immersed Apertures. 



(A Ri'xHy to Col. Dr. Woodward.) 



By F. H. WENHA.M, Vice-President E.M.S. 



My best thanks are due to Col. Woodward for the handsome way 

 in which he acknowledges in the Journal of last month any services 

 of mine for the improvement of the microscope. I can assure him 

 that it is a real pleasure to discuss a subject with one who writes 

 ■with such ability and candour, even when we must necessarily 

 differ in our views. I desire the result sought for without preju- 

 dice, and will test it by practice as soon as the conditions are satis- 

 factorily determined. 



This interminable aperture question commenced three years 

 ago, and anyone taking the trouble to track back (I cannot now 

 find time) will see that it began by my controverting a statement 

 that there was no loss of aperture by immersing the front of the 

 objective in fluid media. It is needless to review, the particulars of 

 my position, which is now practically acknowledged in tliat respect. 



After a period of quiescence the controversy has now opened 

 again, and. Col. Woodward's letter requiring notice, I have again 

 to reply. He first naively says that after some hints given, he 

 sincerely hoped that I would have come to the same conclusion 

 that he has done. For this I do not blame him, having a weakness 

 this way myself. 



I cannot think that I have anywhere stated distinctly that it 

 was not possible to construct an object-glass with an immersed angle 

 exceeding 82^ ; for I wrote on this subject near twenty years ago, 

 demonstrating the loss, and actually constructed and described a 

 combination that gave the full aperture with improved definition ; 

 it is therefore futile to bring this against me, as I at once concede 

 the full aperture on an immersion system specially designed for the 

 purpose, as that was. The front lens may act either positively or 

 negatively, or be neutral, according to its position. 



Col. Woodward now proposes to show that there is " no theore- 

 tical difficulty" in obtaining a balsam aperture of 100", and kindly 

 tenders for enlightenment a diagram to make the way clear, for 

 which I thank him, and having taken it into consideration, let 

 me first apply a sentence that he has made handy for me, thus : 

 " This question I respectfully submit to my friend Col. Wood- 

 ward is not one that can he, decided a ])riori by considerations 

 of optical law. The devices of ingenious workmen are generally 

 hept secret!' The truth of the first part of this statement I will 

 take upon Col. Woodward's own diagram. In the last particular 

 he labours under a disadvantage, perhaps, not shared by myself; 

 for should I venture to demonstrate a principle, I will give the 



