A Top Stop for the Microscope. By J. W. Gordon. 11 



exact phase when cross light throws one half of the mountain into 

 shade, and its shadow on the moon's disk. With an object on the 

 stage of the Microscope it is otherwise. You have command of its 

 illumination, you can produce cross lighting at will, and invoke the 

 precise phase that suits your immediate purpose. Furthermore, 

 the top stop enables you to do this not only without sacrifice of 

 angle, but with an actual increase of effective angle in your 

 objective and consequently with undiminished resolving power. 



I pass now to the last specimen with which I propose to 

 trouble you this evening. Plate III. fig. 7 comprises two photo- 

 graphs of a specimen of Staphylococcus, taken, the upper one witli- 

 out, the lower with the aid of a top stop. Plate III. fig. 8 is a 

 drawing of the same object, made for the purpose of exhibiting 

 certain features too minute to be photographed with the low magni- 

 fying power, which, as already explained, I was constrained to 

 use. The scale imposed upon the photographs reads in degrees 

 of jooWo ^"•' ^^^ ^^^^ marginal photograph shows the Eamsden 

 circle with the stop in position. 



Tlie upper photograph exhibits nothing remarkable except the 

 fact that the negative has stood perfectly well an enlargement of 

 over twenty fold, for it will be seen that as here printed the picture 

 has a magnification of nearly 7000 diameters. But, save for its 

 large scale, it is precisely what most photographs of Staphylococcus 

 are, a picture of little spherical masses of pigment accompanied by 

 a very faint and vague indication of an enveloping sheath. 



The second picture — formed by the stopped lens — is of a totally 

 different character, and unless somebody else has been using a top 

 stop, I imagine that no such detailed picture of a Staphylococcus 

 has ever before been seen. The stop has a semi-aperture of • 65 /, 

 and the objective annulus has accordingly a semi-aperture of 

 1 • 65 /, and yields accordingly an antipoint of the dimensions which 

 would be produced by a lens of N. A. = 1-65, if such a lens could 

 be made. But it is not the large angle of this combination which 

 yields the remarkable detail of the second photograph. The 

 resolving limit has in these pictures nothing whatever to do with 

 the objective aperture, but is settled by the diffusion circle of the 

 camera. Hence the necessity of a sketch to show the full resolu- 

 tion obtained. The stereoscopic representation is secured by 

 selecting the stop and adjusting the condenser aperture so that the 

 widely refracted light may show us the object under a large angle 

 of observation. 



There are here several points to which attention may be 

 directed. Most remarkable of all is the very striking display in 

 the second picture of the enveloping jelly — the sheath— which in 

 the first picture is all but invisible. This sheath is generally to be 

 seen in the images of bacteria formed by lenses of wide angle, but 

 as a rule only as a bright edging to the outline of the object. It 



