( 221 ) 



III. — Final BemarJcs on Immersed Apertures. 



By F. H. Wenham, V.P.K.M.S. 



In closing tliis discussion I have to thank Col. Woodward for 

 ohligiugly furnishing a diagram. When I requested this I did not 

 expect the forthcoming tangible proof by direct measurement of the 

 angles of the |th belonging to Mr. Crisp, made by Mr. Tolles ; this 

 having decided against the alleged extra aperture, the argument 

 may also end with him. Some, however, still follow and uphold 

 these ultra rays on theoretical grounds, and caU for a notice which 

 may be brief. 



Mr. Keith's illustration in the September number of this Journal 

 refers to a xV^^- -'- ^^^^ ^^^t raise any question of its accuracy as a 

 mere diagram, as I doubt the first position taken, and the direction of 

 the rays that follow. I could give in diagram a dry lens admitting 

 the largest pencil possible, that when immersed would fall within the 

 theoretical limit, and so argument might be continued ad infini- 

 tum, and quite uselessly, as we have the object-glass referred to in 

 this controversy to settle the point by measurement. 



It will be seen in the diagram that while the back lenses may 

 assimilate for a roth, the front is of small radius and diameter, 

 magnifying sufficiently for a xV or xV- This is very different in size 

 from the " unfortunate " tenth previously sent also to prove the extra 

 immersion rays. I am not prepared to allow the front to be correct 

 in size or position with its radiant point " assumed" as stated.* If 

 it were moved into the position necessary for a dry object the focus 

 would fall on or within the front surface, and in any position there 

 will be no air focus. If the diagram is made up of uncertain 

 measurements, what is the use of it ? Mr. Tolles, from whom all 

 dimensions come, has repeatedly supplied diagrams to suit his 

 theory. On page 14 of this Journal for July last he assumes that 

 in all conditions of the combined lenses an angle of 60° can be 

 obtained from the back systems. He there places the hemispherical 

 front in this angle just where it fills it best, and at an exceedingly 

 long distance from the middle. On the other hand, in Mr. Keith's 

 illustration it is brought very close ; a contrast indeed to the former 

 case, where the increase of aperture is to be obtained by actually 

 separating the lenses.f 



Of course Messrs. Woodward and Keith are not responsible for 

 data, but the former gentleman tells us it is " a diagram accurately 

 constructed in accordance with the comjputed results": X having, 

 therefore, been drawn to suit the proposition, it may be dismissed. 



Referring back to Mr. Keith's correspondence, I find he first 



* ' M.M. J.,' Sept., 1874, p. 124. t Ibid., July, 1874, p. 14. 

 X Ibid., Sept., 1874, p. 126. 



