17 



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-piece 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 muslin 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 they 

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

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

 ture is made to form an image of it in the principal 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 

 traverses as the i - adius of a graduated circle, having its centre corre- 

 sponding with the place of the original object, and therefore with 

 the image to be viewed ; and the angle of aperture is measured by 

 the arc passed through between two extreme positions, in the usual 

 manner. The method is further explained in the paper by a figure 

 and description of the apparatus, which was itself exhibited in the 

 Library after the meeting. 



VOL. VII. C 



