ANGLE OF APERTURE. 



35 



zontal graduated circle, and is adjusted so that the point of a needle 

 made to coincide with the axis of motion of the movable arm, may be 

 in focus and in the centre of the field of view. The other microscope, 

 to which the object-glass to be examined is attached, is fixed, and so 

 adjusted that the point of the same needle may be in focus in the 

 centre of its field. The eye-piece of the latter is then removed, and a 

 cap with a very small aperture is substituted, close to which a lamp is 

 placed. It is evident that the rays transmitted by the aperture will 

 pursue the same course in reaching the point of the needle as the 

 visual rays from that point to the eye, but in a contrary direction ; and 

 being transmitted through the movable micro- 

 scope, the eye will perceive an image of the bright 

 spot of light throughout that angular space that 

 represents the true aperture of the object-glass 

 examined. The applications of this instrument 

 in the construction of object-glasses are too nu- 

 merous to be here detailed : amongst the most 

 obvious of which may be mentioned the ready 

 means it presents of determining the nature, and 

 measuring the amount of the aberration in any 

 given optical combination. 



Fig. 30 represents the body of one of Mr. 

 Ross's compound microscopes with the triple ob- 

 ject-glass, where o is an object; and above it is 

 seen the triple achromatic object-glass, in con- 

 nection with the eye-piece e e, ff the plano-convex 

 lens ; ee being the eye-glass, and ff the field-glass, 

 and between them, at bb, a dark spot or dia- 

 phragm. The course of the light is shown by 

 three rays drawn from the centre, and three from 

 each end of the object &; these rays, if not pre- 

 vented by the lens ff, or the diaphragm at b b, 

 would form an image at a a; but as they meet 

 with the lens// in their passage, they are con- 

 verged by it and meet at b b, where the diaphragm 

 is placed to intercept all the light except that 

 required for the formation of a perfect image ; 

 the image at b b is further magnified by the lens 



fig. 30. 



e e, as if it were an original object. The triple achromatic combination 

 constructed on Mr. Lister's improved plan, although capable of trans- 

 mitting large angular pencils, and corrected as to its own errors of 



