Final Remarks on Immersion Apertures. 127 



front discussed in my November paper * was not a complete hemi- 

 sphere posteriorly, but the curve ceased at 78° from the optical axis, 

 or 12^ less than the posterior curve of the front lens of the com- 

 bination discussed by Mr, Keith ; as a consequence the rays pro- 

 ceeding backwards, after having passed through the front, will 

 diverge less from the optical axis for any given balsam angle, in 

 the case of the latter lens, than they do in the case of that figured 

 in my diagram, 



Mr, Keith further states that he has computed the spherical 

 aberration of the combination, adjusted as above, and finds it prac- 

 tically nil. This being the case the objective ought to perform well 

 when adjusted to the point of maximum aperture, if balsam be used 

 as the immersion fluid in lieu of water, and the thick cover ordina- 

 rily employed at this position of the screw-collar. Accordingly, in 

 company with jMr. Keith, I tested the objective in this way on 

 Grammatophora suhtilissima by lamplight, and we both thought 

 the definition unmistakably better than with water immersion. 



Mr. Keith has not considered it important to discuss the slight 

 chromatic aberration which this combination is admitted to possess, 

 because it was constructed with special reference to freedom from 

 spherical aberration when used with monochromatic sunlight. I 

 may say, however, that this residual chromatic aberration is not so 

 great as to interfere in the least with the definition of the objective 

 when used with white-cloud illumination or lamp. 



I have also to record of this objective, that although it was con- 

 structed for use as an immersion lens only,t yet I have recently 

 found by trial, that if the screw-collar be turned nearly to the open 

 point, it performs admirably when used as a dry lens, on objects 

 mounted dry, as well as on those mounted in balsam, provided the 

 covers selected are of suitable thickness.^ 



Let me now remind my friend Mr. Wenham of his recent 

 promise on the subject of this controversy. " I should have been 

 glad if Col. Woodward had given us an illustrative figure, having 

 some relationship to a reality with the rays carried to their final 

 destination. The passage of all his rays should have careful con- 

 sideration, and if I saw no error I could not state that there is one, 

 and trust that I have the candour to admit accuracy." § 



With this I dismiss the subject ; nor can I be expected to pay 

 attention hereafter to the assertions of anyone who may continue to 

 hold that it is " theoretically impossible " to construct immersion 

 objectives with a balsam aperture greater than 82° " of image-form- 

 :ng rays," unless he can show some material error in Mr. Keith's 

 computations. Before I conclude this paper, however, I feel called 



* This Journal, November, 1873, p. 211. 



t Ibid., p. 214. 



X Sec Mr. Wenham's remarks, ibid., March, 1874, p. 119. 



§ This Journal, April, 1874, p. 171. 



