79 



On the Influence op Diffraction in Microscopic Vision. 

 By Frank Crisp, LL.B., B.A., Sec. R.M.S., V.P.Q.M.C. 



{Read June 28, 1878.) 

 Plate VII. 



The subj ect which I bring before you this evening is now, so far 

 as date is concerned, somewhat of an old story, but unlike some of 

 the other old stories of the world which are being daily repeated it is 

 one which, I think, will peculiarly bear repetition — the discoveries 

 of Professor Abbe, in relation to the resolution of minute structure, 

 being as remarkable as any that have been made in ancient or 

 modern Microscopy. 



I intend to deal only with the most elementary parts of the sub- 

 ject, advisedly abstaining from attempting to present any more com- 

 plete resume, as I am convinced that its real significance and import- 

 ance will be far more likely to be appreciated if I confine myself to 

 the leading points. Moreover, there will be this advantage, that I 

 shall have left ample room for entertainment on future evenings, and 

 every one will be able to leave to-night with the knowledge that he 

 can occupy very many leisure hours in further investigations. 



I shall not, perhaps, be far wrong if I assume that there are some 

 present who are not aware that within the last two or three years 

 the theory of microscopic vision has received a new and important 

 development, so that some of the old notions — which seemed to 

 depend on the simplest principles, and not to be susceptible of 

 innovation — have been completely exploded, leaving a feeling of not 

 a little wonder that such simple facts had escaped observation, and 

 that we should have had them so long, literally right under our 

 noses, without any appreciation of their importance. 



It is desirable at the outset to point out the two leading ideas 

 hitherto held, which are now known to be erroneous. 



The first error relates to the mode of formation of the microscopic 

 image. It will be remembered that in our books on optics or the 

 microscope, the images of objects are stated to be formed in that 



