Further Bemarks on Immersion Apertures. 119 



Given an object-glass of extreme available aperture, place a plate of 

 parallel glass in front, although the focal point is carried farther 

 off, yet the ultimate aperture beyond the glass remains the same. 

 Insert water between the two, and the focus is still further extended, 

 but the final maximum aperture remains as before, and is neither 

 increased or diminished. Now, with the object-glass the same as at 

 first, set the focus on a balsam-mounted object; the angle of the 

 radiant pencil from this for extreme emergence from the shde 

 must be limited within the maximum of 82°. After refraction, 

 the rays enter the front of object-glass, and this identical law of 

 refraction again limits the internal angle to 82^, the same as in 

 the slide. Now introduce fluid between the two (say balsam as 

 before) ; the focus becomes extended, by which the bend that existed 

 between the two dry surfaces is drawn out straight, if I may so term 

 it, and the lines of similar angle that existed in the slide, and cor- 

 respondingly in the front lens, are now unbroken, and continue entire 

 from the radiant point to the back surface of fi'ont lens. 



In the early part of this controversy, it was contended that the 

 whole aperture could be obtained with an immersion lens of ordinary 

 construction on balsam objects. Even with the utmost stretch that 

 opposition can favour, there has been a great reduction on this 

 assumption, and with the gross errors that may arise, and to which 

 I have now called attention, in the measurement of all extreme aper- 

 tures, I maintain that uo aperture consisting of image- forming rays 

 beyond 82^ can be seen, if measured properly and with freedom 

 from prejudice, in an ordinary object-glass without additions, and 

 that can also be used on dry objects, or such as are not mounted in 

 balsam, which include the majority of tests. 



V. — Further Remarks on Immersion Apertures. 



By J. J. WooDWAED, Assistant-Surgeon U. S. Army. 



I AM glad that Mr. Wenham finds " a real pleasure " in discussing 

 this question with me, since I have somewhat more to say, this 

 time in the shape of comment on the subject of his " reply " in the 

 December number. 



In the first place, I might easily cite passages from his former 

 papers to justify my understanding that he distinctly denied the 

 possibihty of constructing "an object-glass with an immersion 

 aperture exceeding 82°," but I willingly admit that I misunder- 

 stood his meaning, since he now says this was not his intent, and 

 that he at once concedes " the fuU aperture in an immersion system 

 specially designed for the purpose as that was." 



VOL. XI. K 



