1898.] MICROSCOPICAL JOURNAL. 93 



but there are obvious points of contact between it and 

 the other, which also have to be considered, and which 

 throw much light upon it. 



2. With the purely theoretical bearings of this recent 

 presentment of the case it is not necessary to deal at 

 length. Yet it seems desirable to mention some objec- 

 tions which suggest themselves, and which, if valid, 

 have much bearing on the conclusion of Prof, Abbe, that 

 " diflFracted light [defined as " light which advances in 

 other directions than those prescribed by geometrical 

 optics]" is the machinery by which good definition is 

 brough about." That is, of course, getting back to the 

 " spectrum " theory ; and this theory is only true in a 

 conditional and limited sense, while its acceptance in a 

 universal sense is a present cause of positive mischief. 



3. There are then, some fundamental physical objec- 

 tions to that method of representing what takes place. 

 Putting it most briefly : (A) All light emitted by an 

 object may be resolved into undulations consisting of 

 uniform plane waves. (B) we may conceive these re- 

 versed in direction (since any dynamical system may be 

 reversed); and when they thus arrive back at the posi- 

 tion occupied by the original object, they will there 

 " produce an image the most perfect that the light 

 emitted is capable of producing." This is held to follow 

 because the plane waves there, as at each step, " reproduce 

 exactly the same state of the ether as had prevailed at 

 the same stations on the outward journey." Hence in 

 general •' plane waves converging inwards " are capable 

 of producing the most perfect attainable image produ- 

 cible from the rays grasped by the objective. Stating 

 objections to this with similar generality and brevity, it 

 appears that such a presentment of the matter raust break 

 down as a full and complete explanation, howernr true in a 

 limited sense, on the ground that " uniform pbiiie waves " 

 such as are spoken of, are not in trustworthy microscopy 



