124 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



tlie 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 substage, 

 no additional arrangement is required besides the diaphragm carrier. 

 Thus, for example, if a Collins' condenser fitting in a rotating substage 

 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 3-4ths in. diameter, in which 

 cu'cles of card may be drojiped, the card being pierced with holes of 

 different sizes according to the directions given above. 



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 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 objectivt) 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 a rotating substage it is 

 sufficient to mount the condenser on a piece of tubing, which may slide 

 in the setting always provided for the diaphi'agm 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 object. 



For this a prepared plate is required which shall present 

 sharply defined black and white stripes, 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 trans- 

 mitted through it. A test plate sujficiently 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. Cover- 

 glasses of various thicknesses, from 0"24 mm. to 0'09 mm. (accu- 

 rately measured), are ruled on one smface thus coated with a film of 

 metal, the groups of lines varying from l-250th to l-1250th in. ; the 

 ruled side is then cemented with balsam on a polished glass slip, 

 several such prepared glasses being cemented side by side on the 

 same slip, presenting the appearance shown in fig. 18 (natural size), 

 fig. 19 being one of the circles enlarged. 



A perfectly corrected objective, tested with the test object, and by 



