GO 



May 11, 1854. 



The EARL of ROSSE, President, in the Chair. 

 The Right Hon. Lord Ashburton was admitted into the Society. 

 The following paper was read : 



" 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. 

 Communicated by ARTHUR HENFREY, Esq., F.R.S. Re- 

 ceived April 29, 1854. 



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 



