A Top Stop for the Microscope. By J. W. Gordon. 13 



what could be rendered separately visible by any optical means 

 whatever ; and although the recent investigation of this subject by 

 Lord Eayleigh has shown the question to be less simple than 

 Helmholtz supposed, and the ultimate limit of resolving powet an 

 object very much smaller than he determined, still the matter rests 

 at present in such a state that it is of interest to accumulate the 

 experimental evidence of resolution that transgresses Helmholtz' 

 rule. The top stop makes this an easy task. 



It will, I hope, have been clear to you that the imperfections of 

 the photographs by which I have sought to illustrate this subject 

 are not due to want of effort on my part to provide something that 

 would be worthy of your consideration, but to the somewhat hard 

 conditions under which I have been compelled to work. My hope 

 is that, with all their shortcomings, they will afford evidence 

 sufficiently striking of what this new appliance may be expected to 

 accomplish, when it gets into more capable hands than mine, to 

 secure adequate consideration for what, if I am at all able to judge, 

 are exceedingly promising lines both of theoretical research and 

 practical advance. 



Note.— Tlie very unsatisfactory picture which stands as fi^. 5 in plate II. 

 requires an explanation and an apology. The small scale and a feeble nega- 

 tive explain its shortcomings. The reason why the scale is so small and the 

 negative so feeble— which must stand for its apology— is that the making 

 of a photograph of so small an object with a beam of so narrow angle is a 

 task of considerable dittieulty. For writing the text and for exhibition at 

 the meeting I used prints made by direct photogi-aphy. Had I realised how 

 much the result would be deteriorated by photo-process reproduction, I would 

 have chosen some bolder object. It is nevertheless jjossible to see in the 

 picture, as it stands, that the higher edge of the mid-rib, for example, is 

 (lark in fig. 5 though bright in fig. 4. The original negatives are full of such 

 contrasts. — J. W. G. 



