58 Royal Society. 



of some of the border tribes that occupy a sort of neutral, and not 

 yet undisputed, ground between the confines of the animal and vege- 

 table kingdoms ; as for example the Isthmia, which possesses a reti- 

 culated structure, with depressions between the meshes, somewhat 

 analogous to that which would result from pasting together bobbin- 

 net and tissue paper. 



The valves of P. angulation and other similar objects have been 

 by some writers*" supposed to be made up of two substances pos- 

 sessing different degrees of refractive power ; but this hypothesis is 

 purely gratuitous, since the observed phenomena will naturally re- 

 sult from a series of rounded or lenticular protuberances of one 

 homogeneous substance. Moreover, if the centres of the markings 

 were centres of greatest density, if in fact the structure were at all 

 analogous to that of the crystalline lens, it is difficult to conceive 

 why the oblique rays only should be visibly affected. When P. hip- 

 pocampus or P. formosum is illuminated by a Gillett's condenser, with 

 a central stop placed under the lenses, and viewed by a quarter-inch 

 object-glass of 70° aperture, both being accurately adjusted, we may 

 observe in succession, as the object-glass approaches the object, first 

 a series of well-defined bright dots ; secondly, a series of dark dots 

 replacing these ; and thirdly, the latter are again replaced by bright 

 dots, not however as well defined as the first series. A similar suc- 

 cession of bright, dark, and bright points may be observed in the 

 centre of the markings of some species of Coscinodiscus from Ber- 

 muda. 



These appearances would result if a thin plate of glass were studded 

 with minute, equal and equidistant plano-convex lenses, the foci of 

 which would necessarily lie in the same plane. If the focal surface 

 or plane of vision of the object-glass be made to coincide with this 

 plane, a series of bright points would result from the accumulation 

 of the light falling on each lens. If the plane of vision be next made 

 to coincide with the surfaces of the lenses, these points would ap- 

 pear dark, in consequence of the rays being refracted towards points 

 now out of focus. Lastly, if the plane of vision be made to coincide 

 with the plane beneath the lenses that contains their several foci, so 

 that each lens may be, as it were, combined with the object-glass, 

 then a second series of bright points will result from the accumula- 

 tion of the rays transmitted at those points. Moreover, as all rays 

 capable of entering the object-glass are concerned in the formation 

 of the second series of bright focal points, whereas the first series 

 are formed by the rays of a conical shell of light only, it is evident 

 that the circle of least confusion must be much less, and therefore 

 the bright points better defined, in the first than in the last series. 



If the supposed lenses were of small convexity, it is evident that 

 the course of the more oblique rays only would be sensibly in- 

 fluenced ; hence probably the structure of P. anc/ulatum is recognized 

 onlv by object-glasses of large angular apertures, which are capable 

 of admitting very oblique rays. 



The writer has recently, in an address to the members of the 

 * Vide Quarterly Journal of Microscopical Science, No. V. pp. 9, 10. 



