PROFESSOR ABBE’S METHOD OF TESTING OBJECTIVES, DoT 
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 circles of card may be 
dropped, 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 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 direc- 
tion 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 diaphragm on the 
under side of the stage. Card diaphrams 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 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. Cover-glasses of various thicknesses, from 0°24 
mm. to o‘og mm. (accurately measured), are ruled on one surface 
thus coated with a film of metal, the groups of lines varying from 
1-250th 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. 
