ON OPTICAL QUALITY OF MICROSCOPE OBJECTIVES. 9 



Another plan adopted by the writer 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 

 substage. 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 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 substage the 

 whole face of the lens is swept and thus searched in any direction 

 required. 



When an instrument is not provided with rotating substage, 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 third 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 the field, and thus permit each 

 part of the central or peripheral zones to be brought into play. 



3. Test Olject. 



For this a prepared plate is required, which shall present 

 sharply defined black and white stripes 5 opaque and clear lines 

 alternating at close intervals, and lying absolutely in the same 

 plane, so that no deviation can occur in the course of pencils of 

 light transmitted through It. A test plate sufficiently perfect for 

 all practical purposes, may be made by ruling groups of lines, 

 coarse and fine, with the aid of a dividing machine on a metallic 

 film of silver or gold of infinite thinness, and fixed by known 

 methods on glass. Covering glasses of various thicknesses 

 (accurately measured) are ruled on one surface thus coated with a 

 film of metal, the groups of lines varying from ai-o^^ ^^ I'iru^ 



