CORBESPONDENCE . 



153 



now exactly under the couditions of an object mounted in balsam, and 

 thus somewliat differing from my trial of two years ago, yet the aper- 

 ture came out the same, viz. 68°. 



Next the polished surface of front lens was again measured by 

 micrometer ; it was foimd, as before, • 043 inch. The distance of the 

 immersion focus on an object in Canada balsam was now carefully 

 ascertained ; the object-glass being properly adjusted for aberration, 

 it was found to be ' 025 inch ; taking the front lens for a base line, 

 with this height the angle is 81|^°, showing that an immersion angle 

 of 98° is simphj impossible. But the utilized portion of the front lens, 

 through which all the rays or aperture emerge or enter, is much 

 within the diameter ; the spot can be ascertained with the greatest 

 precision. With the lenses closed so as to give the largest area, the 

 working diameter through which the rays passed was found to be 

 only '033 inch. 



Mr. Mayall says that he is " compelled to take my utterances 

 in the ' M. M. J.' as representing the views I hold." I believe he 

 has no alternative. I am conscious of some omissions and obscurities 

 in descrii)tion, but had I described everything that I have tried, 

 this already excessively tedious controversy would have become quite 

 intolerable. At page 117 of this Journal for March, 1874, in ex- 

 plaining the use of the semi-cylinder, I say, " And the focal front 

 (meaning point) of object-glass a, Fig. 4, brought to the centre of the 

 semi-cylinder c, at which there is a thin metal slit or stop of suitable 

 diameter." Then follows this sentence : " In this measurement I have 

 rather over than imder estimated the aperture from using a stop too 

 large ; less than g'oth of an inch would have been more proper." There 

 is certainly a mistake here, carelessly written. This misplaced sen- 

 tence might have been left out, as the main strength of Mr. Mayall's 

 argument is based upon it. I told him positively, and now repeat, 

 that in the conical front cap with the J^ inch front opening I never 

 used either water or balsam, and that all the inunersions were tried 

 with a slit, as I had a lively recollection of the trouble of adjusting 

 slits of different widths tacked on with Canada balsam, and set parallel 

 in place under a low power. 



The conical aperture of -^^^ inch was adopted merely as one means 

 of showing that in the ith the preposterous aperture of 180° did not 

 exist. I had not then discovered the slit. The idea of this, and its 

 adoption, was suggested by the conical aperture. 



I am very glad that Mr. Mayall, after the publication of such a 

 statement, suggests a trial before competent judges. I accept the 

 challenge with much pleasure, and hope he will form an unprejudiced 

 committee who will not, at all events, wade back thi-ough all the 

 length of this miserable controversy to seek only for dubious sentences 

 or anomalies of description. The simple facts before them woidd be. 

 Does this glass give an immersion angle beyond 82°, or does it not? 

 on the items of the immersed aperture taken with the slit and the 

 semi-cylinder, and also on the measurement of the diameter of the 

 front lens, and the length of the corrected immersion focus, on an 

 object in Canada balsam. The last alone will suffice to show that the 



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