212 Royal Society. 



very of some structure indicative o!/ albumen or embryo, is abso- 

 lutely essential to the complete establishment of the affinity I have 

 suggested. 



It must not be overlooked, that the characters through which I have 

 attempted to establish an affinity between Trigonocarpon and Coniferae 

 are equally common to the fruits of Cycadese; and in connexion with 

 this subject I may remark, that M. Brongniart* has referred the 

 genus Noggerathia, which is also found in the coal-measures, to 

 that natural order, together with some associated organs which are 

 probably Trigonocarpons in a mutilated state. The leaves of Nog- 

 gerathia are, however, alone known, and Dr. Lindley, when figuring 

 those of one species (Lindley and Hutton, Fossil Flora, 28, 29), 

 pointed out their great resemblance to those of Salisbrtria, thus 

 affording collateral evidence of the view I have been led to adopt 

 from an examination of the fruit alone. 



May 11, 1854. — The Earl of Rosse, President, in the Chair. 



" On the relation of the Angular Aperture of the Object-Glasses 

 of Compound Microscopes to their penetrating power and to Oblique 

 Light." By J. W. Griffith, M.D., F.L.S. 



The explanation given by Dr. Goring and others of the advantage 

 of increased angular aperture in microscopic objective- glasses ap- 

 pears to the author to be correct, as applied to the case of opake ob- 

 jects, and accordingly his remarks in the present communication 

 have reference to transparent objects only. 



It is known that delicate markings on a transparent object, such 

 as the valve of a Gyrosigma , may be rendered more distinctly visible 

 by using an object-glass of large aperture, by bringing the mirror to 

 one side, and by placing a central stop in the object-glass or the con- 

 denser or in both ; the increased distinctness produced in these seve- 

 ral ways being due to the illumination of the object by oblique light. 

 Experiment also shows that the degree of obliquity of the light re- 

 quisite varies with the delicacy or fineness of the markings, being 

 greater as these are more delicate ; so that the finest markings re- 

 quire the most oblique light which can possibly be obtained to ren- 

 der them evident, and the angular aperture of the object-glass must 

 necessarily be proportionately large, otherwise none of these oblique 

 rays could enter it. 



If the parts of an object which refract the light are large in pro- 

 portion to the power of the object-glass and of irregular form, they 

 will refract a certain number of rays, so that these cannot enter the 

 object-glass; hence certain parts will become dark, and will map out, as 

 it were, in the image formed of the object, the structural peculiarities 

 of the same. But if the parts are minute, of a curved form and ap- 

 proximatively symmetrical, they will act upon the light transmitted 

 through them in the manner of lenses, and their luminous or dark 

 appearance will vary according to the relation of the foci of these to 

 that of the object-glass. Thus the parts of an object may appear 



* Annales des Sciences Natuvelles, 2nd Series, vol. v. p, 52. 



