386 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



revolve round the axis of the instrument, and also carry with it 

 the ring or tube to which the card diaphragm is fixed, the pencils 

 of light admitted through the holes will, by simply turning the con- 

 denser round, sweep the face of the lens in as many zones as there 

 are holes. Supposing the condenser to be carried on a rotating 

 sub-stage, no additional arrangement is required besides the 

 diaphragm-carrier. Thus, for example, if a Collins condenser fitting 

 in a rotating sub-stage be used, all that is required is to substitute 

 for the diaphragm which carries the stops and apertures as arranged 

 by the maker, a diaphragm pierced with, say, three openings of |-inch 

 diameter, in which circles of card may be dropped, the card being 

 pierced with holes of different sizes according to the directions given 

 above. We doubt, however, if any sub-stage will revolve with 

 sufficient accuracy for so delicate a test. 



Another plan adopted by Dr. Fripp, and found very convenient 

 in practice, is to mount a condensing lens (Professor Abbe's in this 

 case) upon a short piece of tube, which fits in the rotating sub-stage. 

 On opposite sides of this tube, and at a distance from the lower lens 

 equal to the focal distance of the combinations, slits are cut out 

 through which a slip of stout cardboard can be passed across and 



FIG, 381. 



below the lens. In the cardboard, holes of various sizes, and at 

 various distances from each other, may be pierced according to 

 pleasure. By simply passing the slip through the tube, the pencils 

 of light admitted through the holes (which form images of these 

 holes in the upper focal plane of the objective) are made to traverse 

 the field of view, and by rotating the sub-stage the whole face of the 

 lens is swept, and thus searched in any direction required. But here, 

 again, the spherical aberration of an unconnected condenser would, 

 with an objective of large aperture, cause the oblique pencils 

 under some conditions to pass under the object ; and alteration of 

 focus will not properly alter this at least without a disturbance of 

 the focus of the objective. 



When an instrument is not provided with a rotating sub-stage, 

 it is sufficient to mount the condenser on a piece of tubing, which 

 may slide in the setting always provided for the diaphragm on the 

 under side of the stage. 



Card diaphragms for experiment may be placed upon the top of 

 a thin piece of tube (open at both ends) made to slide inside that 

 which carries the condenser, and removable at will. By rotating 

 this inner tube the pencils of light will be made to sweep round in 



