THE MICROSCOPE. 199 



form a convex (internally concave) surface. The resolution, as any- 

 one can easily convince himself, is principally effected by a very 

 oblique ray of light striking the object (very obliquely) from above, 

 as shown in the drawing by the lines, marked a, a, so that if 

 all other rays, those indicated by the other lines, marked b, b, b, 

 and the diffused'light in the room could be excluded, the field would 

 present a dark ground illumination. 



I communicated my discovery, if such it can be called, to Mr. 

 R. B. Tolles, and he fully indorsed my views, as the accompanying 

 letters from Mr. Tolles will show. Still, he too, makes a trifling mis- 

 take in assuming that the curve formed by the edge of the immer- 

 sion fluid will be much flatter than represented in my drawing; if it 

 is much flatter, the resolution, as will be easily seen, will be effected 

 by a line more central than shown in my drawing, which, by 

 the way, is the same I sent to Mr. Tolles in explanation of my 

 find. The flatness or steepness of the curve formed by the 

 edge of the immersion fluid depends upon the stiffness of the 

 fluid, the amount that is used, and, above all, upon the 

 working distance of the objective. If the latter is rather 

 short, like that of my ^ homog. immers, of Tolles and of my -fa 

 homog. immers, of Zeiss, even the curve formed by cedar oil is suf- 

 ficiently steep to effect a most beautiful resolution in the way indi- 

 cated in my drawing, if only the light is so directed that the edges 

 of the same, as reflected from the mirror, will strike that curve. If 

 one wishes to try the experiment, I would advise them to place 

 the microscope in a position in which the direct rays of the sun can- 

 not fall on the stage. What Prof. A. Y. Moore says in the June num- 

 ber of- The Microscope, about collar-correction is mere bosh to me, 

 or at least too silly to deserve any reply, and his explanation given 

 of the means by which the resolution is effected, unless intended as 

 a joke, is untenable and contrary to all optic laws. At any rate the 

 lower surface of all my objectives 'made by Tolles, Zeiss, Bausch & 

 Lomb, and Hartnack, is well polished; that of my Tolles' ^ is 

 beautifully laquered and very smooth, that of the j 1 ^ Tolles' is 

 heavy gold plated, and perfect in its finish, and that of the Zeiss' T *j 

 is nickel, or nickel-composition, and exceedingly well polished. 

 Neither of them contains convenient holes capable of reflecting the 

 light where wanted. 



In order to make Mr. Tolles' last letter, marked No. 2, inteU 



