16 



March 9, 1854. 

 THOMAS BELL, Esq., V.P., in the Chair. 



The following paper was read : 



" On a new and more correct method of determining the Angle 

 of Aperture of Microscopic Object-Glasses." By WILLIAM 

 S. GILLETT, Esq., M.A. Communicated by CHARLES 

 BROOKE, Esq., M.A., F.K.S. Received March 9, 1854. 



The very large apertures assigned to the more recent microscopic 

 object-glasses drew the author's attention some time since to the im- 

 portance of testing the accuracy of the method employed to deter- 

 mine their amount. 



With this object in view he began with the consideration that 

 the central pencil was alone to be regarded, and that the marginal 

 rays of this were the true limits of the angle of aperture, and that 

 consequently the rays of all oblique pencils were to be excluded, as 

 these might cross at a point not coincident with the principal focus, 

 and being measured separately might form an angle (apparently of 

 aperture) not coinciding of course with the true one, although per- 

 haps not differing from it in amount. A short description of the 

 usual method of measuring these angles will suffice to show what 

 claim it has to confidence in these respects. 



The microscope, with the object-glass to be examined and an or- 

 dinary eye-piece, is used as a telescope, and a light placed at some 

 distance is commonly made an object to define the limit of the field 

 of view, the image of which is formed near the back surface of the 

 posterior combination, and the diffused light of this image, as seen 

 through the eye-piece, is the indication that a pencil of light is ad- 

 mitted, whether central or oblique. Sometimes by an additional 

 glass the eye-piece is made an erecting one capable of bringing the 

 image into focus. This adds much to the convenience, but not to 



