Tlie Abbe Diffraction Theory. By J. W. Gordon. :395 



light, so that the mounting and sub-stage arrangements will be much 

 simplified. The one thing that it will not do will be to gather in the 

 diffracted rays scattered by minute striae in the object ; but it will be 

 found that this deficiency will not impair its performance. 



Another appliance intended to serve the same purpose is shown in 

 fig. 94. This, perhaps, can hardly be called an objective at all ; it is 

 rather an adjunct to an objective, and is intended to be placed behind 

 the objectives in present use for improving their definition and 

 resolving power. It consists of a negative lens for expanding the 

 wave- front, backed by a positive lens of counter-equivalent power 

 for restoring the focus. In this way a back lens of much larger 

 diameter than could be conveniently mounted in 

 an objective of standard size can be worked up Fig. 94. 



into the combination, and a proportionate im- 

 provement in the performance of existing ob- / 

 jectives may be looked for. 



At present these are only theoretical results, 

 and it may be that many practical difficulties will 

 have to be surmounted before they can be realised 

 in practice. But they point out one direction 

 in which marked improvements in objective con- 

 struction may reasonably be looked for — provided, of course, that a 

 superstitious adherence to the Abbe theory does not prevent the 

 Microscope makers from trying experiments in these directions. It 

 is out of my power to put these theoretical results to the rigid test of 

 experiment ; but I may say, for the encouragement of others more 

 fortunate in this respect, that I have made some experiments with 

 rough models improvised out of the lenses of opera-glasses, with 

 results that appear to me, and, I believe, to some of my friends, to be 

 highly promising. 



It is perhaps unnecessary, but I should like to guard myself by 

 adding that these suggestions for the improvement of objectives are 

 not only theoretical — the drawings are mere diagrams and in no sense 

 designs. I am far too sensible of my limitations to offer suggestions 

 to the Microscope makers concerning the designing of their instru- 

 ments. My task will be accomplished if I succeed in drawing 

 attention to the possibilities of this type of combination. 



There is one more word which, as a visitor, I hope I may be allowed 

 to add — a word in acknowledgment of the very liberal way in which your 

 Society has opened the pages of its Journal to the publication of this 

 paper, and placed its abundant facilities of illustration at my disposal, 

 for the purpose of exhibiting the argument which I have now the 

 honour to submit for your consideration. I do not presume to make 

 that acknowledgment in the form of thanks, for I am well aware 

 that this very effective support is not conferred upon me as a boon, 

 •but is extended to me quite impersonally as the exponent of a view 



