A Top Stop for the Microscope. By J. W: Gordon, 7 



beam, and when that edge is cut off by the diaphragm interposed 

 in the optical system which produces fig. 1, the dot is accordingly 

 darkened — that is to say, it is darkened by the diaphragm. More- 

 over, it is darkened in so much greater measure than the field that 

 it shows up as a black dot. But when the stop is introduced in 

 the system of fig. 2, the refracted light from the dot in the diatom 

 passes round it, suffering very little diminution, and the dot itself 

 appears accordingly as a bright object in a dark field, for the 

 unrefracted field light is sensibly diminished by the stop. 



Tlie explanation of the delicate tracery upon the field of fig. 2 is 

 similar. The surface of the diatom is not a true plane like a piece 

 •of optically-wrought glass, but an undulating surface like the 

 surface of bottle-glass. Hence, slight refractions are produced at 

 every point, and these vary from point to point the capacity of the 

 emitted liglit for passing round the stop, and so give rise to these 

 exquisitely delicate indications of structure. This view of the 

 function of a stop in exhibiting the most delicate forms of 

 diaphanous structure was originally put forward, so far as I am 

 aware, by Tcippler, and may be found very clearly explained in his 

 paper on the subject in Poggendorfs "Annalen" for the year 

 1867.* 



If, now, you examine fig. 3, you will observe that it is a blend 

 of figs. 1 and 2, but it is a blend in which the distinctive features 

 of each are weakened by the admixture of the other. Thus the 

 tracery has almost disappeared from the smface of the diatom, and 

 the black dot has lost solidity and acquired a faintly Ijright 

 centre. 



In this case, then, the answer to the question just proposed 

 appears to be undoubtedly that the wide-angled annulus and the 

 narrow-angled core mix, but do not combine their powers, in the 

 full-beam picture. If we want to see all tliat the wide-angled 

 component of this beam can reveal we must suppress the centre ; 

 and on the other hand if we want to study the picture limned by 

 the centre of the lens we must cut off its margin. Perhaps it will 

 be thought that this last remark is not worth making. And indeed 

 it is obvious that the angle of any lens can be cut down by a 

 diaphragm. But what is worth pointing out in this connection is 

 that the picture obtained from a narrow beam has a value of its 

 own. Too much, or at least too exclusive,. attention has in recent 

 years been devoted to the merit of wide-angled lenses, so that 

 probably it will savour of paradox if I presume to say that the 

 one picture is a useful and in some cases a necessary supplement 

 to the other. And as that is what I desire to say this evening I 

 propose to entrench myself in fig. 1. Confessedly fig. 2 shows 

 much more of the structure than fig. 1, and if we had to choose 



* Pogg. Ann., cxxxi. p. .S3. 



