Immersion Lenses and New Mefradometers. 75 



mercurial image from the globule placed immediately under deep 

 objectives is im.]30ssible. 



I cannot properly conclude this article "without some allusion to 

 the edifying and philosophical personalities which grace Mr. W.'s 

 papers. I can only ask the readers of this Journal to decide 

 whether he has forfeited the pledge he gave to carry on the con- 

 troversy "in a fair spirit, being willing to receive or give any 

 information that may tend to elucidate truth" (p. 301, June, 1870). 

 [The "information" would surely have been quite as valuable 

 without the ornate argumentum ad Ziommem so liberally employed.] 

 His chief reason for commencing the controversy being, as he states 

 in the same page, " the slur that is cast upon the object-glasses of 

 our best makers by the assertion, that in the best glasses there is a 

 residuary aberration which obscures the clear definition under a power 

 of 1000." It is fair to compare this with the new admission. 

 Speaking of my double-star test, he says (January, 1871, p. 22), 

 " A had glass ivill hlur them together, and a good one ivill separate 

 them more or less distinctly" Do any of the old-fashioned glasses 

 or Mr. Wenham's best objectives blur them ? 



" As no optician thinks of the possibility of a distinct image " 

 whilst using the mercurial globule at present, I entertain a lively 

 hope that the time is not far distant when they will use the image 

 tests such as I have elsewhere described, as an exquisite means of 

 detecting unsuspected residuary errors. Mr. Wenham, however, 

 seemed at one time well aware of the many difficulties attending 

 the construction of object-glasses, as the following passage fully 

 demonstrates : — 



" I am of opinion the case is different with a microscope object- 

 glass, wherein, with the highest powers, every trijiing error is 

 enormously magnified " (p. 228, V.). 



On the other hand he declares, " the efiect of the water and 

 covering glass is precisely the same in its corrective action as 

 additional thickness thrown on the front lens ! ! ! " The same very 

 singular idea runs through the last efiVision of Mr. Wenham. 



It is, however, tolerably well known among physicists that the 

 effects of irrationality on chromatic dispersion vary very consider- 

 ably according as glass or water is the refracting medium,* and the 

 thicknesses under use. 



I feel quite certain that the best makers of English immersion 

 lenses wUl hardly endorse either this adventm-ous statement, or 

 another, to the effect that " from an |th upwards perfect correction 

 can be obtained from a single front." 



I am greatly impressed with the admission that the chief use of 

 the globule star is to examine the intended errors of the parts, and 

 not the requisite performance of the complete objective. 



* See art. " Light," ' Ency. Met.' 

 VOL. V. G 



