Royal Society. 369 



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 

 the correctness of the method. Thus the conditions of the micro- 

 scopic object-glass are reversed, the principal focus being transferred 

 from the front to the back, and the rays estimated are those of the ex- 

 treme oblique pencils, which may or may not pass through the point 

 of the principal focus of the glass when used for the microscope. 



The importance of this in the illumination of objects immediately 

 suggested itself ; and the author obtained a further proof by another 

 experiment bearing directly upon this point. A blackened wire was 

 placed under a microscope at the focal point, with an object-glass 

 of considerable power and aperture, the wire covering the field with 

 the eye-jjiece used. The field was then illuminated with an achro- 

 matic condenser, the field of illumination exceeding, as it usually 

 does, that of the microscope. As was expected, the oblique rays 

 ■which passed on both sides of the wire prevented its blackness from 

 being seen (this becoming of a milky-grey), until the field of illumi- 

 nation was reduced to the extent of that of the microscope, when it 

 immediately assumed to the eye its natural blackness. This re- 

 minded the author of a beautiful illustration given by Professor Fara- 

 day some years since at the Royal Institution, of the effect of glare 

 produced by placing white musUn blackened in parts before a white 

 paper printed in large letters ; with the white muslin in front, the 

 letters were scarcely visible, while through the blackened parts tliej' 

 resumed their natural appearance. These experiments suggested 

 the new method adopted, which may be briefly stated as follows : — 



The microscope of which the object-glass is to be examined is 

 placed horizontally and centred by an object placed in the focus. 

 Next, tliere is substituted in place of the eye-piece, a hollow cone 

 with an aperture at its summit. Light passing through this ajier- 

 ture is made to form an image of it in the i:)rincipal focus of the ob- 

 ject-glass, in the place of the original object. On this image a 

 horizontally placed examining microscope is then directed, which 



Phil. Mag. S. 4. Vol. 7. No. 46. May 1854. 2 C 



