Royal Microscopical Society, 93 



It must be observed that in objectives of high power the reduc- 

 tion of the number of surfaces, some of which are of deep curva- 

 ture and difficult construction, from 16 to 10, is of itself a very 

 great advantage, since the scattering of light, and consequent fog, 

 incidental to repeated surface action, must necessarily be much 

 diminished. It appears very desirable that when the best form 

 and dimensions that experience can dictate have been arrived at, the 

 results, thrown into an algebraical form, should be submitted to 

 the diii'erential calculus, in order that it might be determined 

 whether the results are the best attainable. To this object the 

 writer would willingly render any assistance in his power. 



Some observations upon the angular aperture of an object-glass 

 by Mr. Tolies in air and water and in balsam, have also been 

 contributed by Mr. Wenham as the result of an examination 

 which was made by him in his study, at which the writer was 

 present, and testified that the angles actually found were those 

 which were stated. This has, however, been somewhat disputed by 

 the maker, who claims a larger angle than that which was found. 

 The lens in this instance was properly corrected as a dry lens, 

 and then after measurement in air it was measured in water and 

 then in very fluid Canada balsam without alteration of the adjust- 

 ment. It may be quite possible that if the lens had been re- 

 adjusted so as to give the best image for immersion in balsam, a 

 slightly greater angle might have been obtained ; but this would 

 not have been a fair way of making a comparison, as it is not 

 the mode in which the glass would ever be employed in actual 

 practice. 



Several papers of a more or less controversial character have 

 been written by Dr. Eoyston-Pigott and Mr. Wenham on the 

 vexed question of the Podura markings, about which so much 

 dift'erence of opinion has existed, and on other tests of optical defi- 

 nition ; and it may here be remarked that in these, as in the dis- 

 cussion of all other purely scientific questions, anything approaching 

 to acrimony is alike to be deprecated and deplored. 



The introduction of a combination of lenses between the eye- 

 piece and objective for the purpose of amplifying the power of any 

 given objective was, it is believed, first suggested by Dr. Goring, 

 in his ' Micrographia,' published about forty years ago; an 

 arrangement of this kind constitutes the " Aplanatic image- 

 searcher " of Dr. Pigott, of which the writer has been unable to find 

 anywhere a precise description. No description whatever appears 

 either in our own or the ' Quarterly Microscopical Journal ' ; but 

 on referring to the paper by Dr. Pigott in the 'Phil. Trans./ 

 Part 2, for 1870, it is found to be thus described : — " A pair of 

 slightly over-corrected achromatic lenses, admitting of a further cor- 

 rection by a separating adjustment, are mounted midway between 



