12 Transactions of the Society. 



is so in this case, and Mr. E. M. Nelson, who has been good enough 

 to examine this particular specimen with a lens of N.A. = 1'3, 

 estimates the edge so seen as having a thickness of -^qq^^ in. 

 With ray own lens unstopped the edge is barely discernible, and I 

 am afraid that my photograph does not show it at all. But with 

 the aid of the stop it is conspicuously visible, and it is not only 

 seen as an edging : it is seen also, by virtue of the distortion which 

 it introduces, as a transparent mass enveloping the coccus forms, 

 and interposed between them and the eye. This is particularly 

 observable at the actual centre of the picture. There are four 

 cocci with a very small space between them, through which the 

 field light shines. A full beam even of very wide angle shows this 

 feature quite undistorted, as though the four cocci were lying in a 

 homogeneous medium. But the annular objective shows these 

 central cocci so much distorted by the refraction of the overlying 

 jelly that they may be likened to grapes seen in the substance of 

 a gelee aux fruits. The central space is hardly to be recognised, so 

 broken is the light in passing through the envelope of highly re- 

 fracting material. In fact, this particular adjustment of the illu- 

 mination shows more of the sheath than of the coccus forms which 

 it incloses. 



And now I must ask you to observe what very high resolving 

 power is here developed. In illustration of this point I might call 

 attention to the photograph itself, where the individual coccus is 

 seen outlined by a dark line that is less than yoij'ooo ^^^- ^" thick- 

 ness. But, as I have already said, my camera at the low magnifying 

 power which I was using falls far short of tlie objective in this 

 particular. I take, therefore, by preference, the sketch fig. 10. 

 You will observe that here there is a minute speck seen upon each 

 of the larger and more outlying cocci, not quite in its centre, but 

 drawn a little towards the centre of the mass. These minute 

 specks are quite sliarply defined in the visual picture. What they 

 represent I do not know. I suspect very much that they are 

 optical phenomena and do not stand for any opaque mass in the 

 specimen itself. But whatever they are, they are immeasurably 

 small. Any estimate of their magnitudes must needs be vague, 

 but it is a pretty safe estimate to write them down at something 

 less than tsqooo ^^- '^^ ^^^"^^ ^^^ example the largest coccus. 

 This measures, to the outside of the dark defining ring, a diameter 



of ^0000 ^^^- ■'■^ from this we deduct goooo ^"- ^^^' ^^^ double 

 thickness of this dark ring, and take one-third of what remains for 

 the visible diameter of this minute speck, we arrive at X44V00 ^°-' 

 and when you examine the specimen itself your eye will apprise 

 you that one-third is a full estimate of the diameter of the speck 

 as compared with the bright centre in which it lies. We thus get 

 strong definition of an object whose dimensions were long supposed 

 on authority no less than that of Professor Helmholtz, to be below 



