202 A. A. C. ELIOT MERLIN ON SOME NEW DIATOMIC STRUCTURE. 



merely for the resolution of striae with very oblique light ; but 

 such lenses would not clearly define any object whatever, dia- 

 tomic or otherNvise, with axial illumination. Experiments with 

 the very considerable number of objectives, of all dates, in my 

 possession, including examples by most of the leading English 

 and foreign makers, have shown me that any lens I have ever 

 seen which, with a large working aperture, affords a clean and 

 brilliant image on one class of object will do so on all. The 

 qualities in an objective, now of prime importance to all in- 

 vestigators, are exceeding sharpness of image and brilliancy of 

 contrast with, at least, a | cone. The powers chosen will 

 naturally vary according to the minuteness of the objects dealt 

 with. 



After good definition, but fortunately far less essential, flatness 

 of field is desirable, but is seldom ever really attained with a 

 large W.A. During last summer I examined a lens supposed to 

 be strong in this respect, but found that it was not so. 



In submitting these remarks for your kind consideration I do 

 so well knowing that differences of opinion exist on the subjects 

 touched upon, but I nevertheless claim your kind indulgence, as 

 my views are the outcome of long practical experience. It is 

 the duty of an observer to state the results of his observations 

 without fear, for divergence of view should not be incompatible 

 with good feeling and friendliness, it being certain that all 

 opinions in scientific matters will eventually stand or fall on 

 their merits. 



JovA-n. Qv.ekett Microscopical Club, Ser. 2, Vol. XL, No. (38, A2)ril 1911. 



