ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 811 



behaves like a daisy under a 4-iii. on focal alteration. Such a case is 

 quite inexplicable by a dioptric theory ; but is quite consistent with 

 the views put forward in my last paper. When the back of the objective 

 is examined, it will be seen entirely covered with spectra, so no 

 zonal differences can exist, and consequently focal alterations will not 

 pioduce different images. The above seem to me to be the chief 

 objections to the dioptric theory. 



In conclusion, let me say that the author of the dioptric theory has 

 done excellent work, although according to my own view he has failed to 

 establish his case. First, he has given the most concise and lucid 

 explanation of the interference phenomena that is extant in our 

 language. Secondly, he has given testimony to the fickleness of images 

 derived from a small cone of illumination." 



Mr. L. Wright * also writes on the same subject : — " I have myself 

 very grave doubts if this new theory is correct ; but it is a singularly 

 interesting one. I draw attention to it partly as a proof that speculation 

 is not yet at an end, but chiefly to point out that there is one most 

 simple experiment, easily made, which will determine it with absolute 

 certainty. That is, to silver the hack of the front lens, and then remove 

 the silver from the centre of the back only. The reflection from the 

 margin will be, if anything, rather increased ; and whatever becomes of 

 the theory in question, I believe the expedient may prove of some service 

 as an illuminator of certain objects, and may give valuable resolution of 

 structure by the modification in this point. But the silver will really 

 stop off all but the central pencil, which it will allow to pass unaltered ; 

 and if Prof. Lowne's theory is correct, the 'high' resolution will be 

 unaffected. I hope such an experiment will be made without delay, and 

 it will be well worth while merely as one in illumination, if no one has 

 attempted it before. I am afraid, however, it will demolish the theory, 

 for if the latter be sound, one would say that all lenses with hemi- 

 spherical fronts ought to give equal resolution, irrespective of aperture, 

 which belongs to the hack portion of the lens. This is not the case, and 

 I fear we have yet to find a theory which shall reconcile the undoubted 

 facts with conclusions that seem forced upon us by the phenomena of 

 physical optics." 



In reference to Mr. Wright's suggestion, Mr. Nelson | points out a 

 way in which the experiment may be jjerformed without silvering the 

 front lens of an objective. 



An inch objective with a Lieberkiihn ought to resolve more than the 

 same lens without a Lieberkiihn with transmitted light, sujiposincr the 

 hypothesis to be correct. If the increase of aperture is only useful for 

 illuminating the object by reflected light, and no rays pass through the 

 increased portion to the eye, it is abundantly evident that those conditions 

 are fulfilled by a Lieberkiihn. The experiment can therefore be easily 

 tried. 



Prof. Lowne remarks % on Mr. Wright's paper as follows : — " I fear 

 the theory which I have suggested to account for the efiicacy of laro-e 

 apertures in microscopy cannot be so easily verified or disposed of as 

 Mr. Lewis Wright supposes. Before giving my reasons, I must correct 

 the impression which may evidently be made by an expression of mine 

 and which it was far from my intention to convey. 



* Engl. Mcch., xUx. (18S9) p. 391. f T. c, p. 416. J T. c, pp. 437-8. 



