1 84 



THE AMERICAN MONTHLY 



[October, 



must then be pierced in a card, in such 

 position as will produce images situ- 

 ate in the field as shown by figs. 32 

 and ;^^, and the card is then fixed in 

 its jDroper place below the condenser. 

 If the condenser be fitted so as to re- 

 volve 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 

 condenser around, 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 



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 re- 

 quired. 



When an instrument is not provi- 

 ded 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 

 diaphraghm on the under side of the 

 stage. Card-diaphragms for experi- 

 ment may be placed upon the top of a 

 third piece of tube (open at both ends) 

 made to slide inside that which car- 

 ries the condenser, and removable at 

 will. By rotating this inner tube the 

 pencils of light will be made to sweep 

 around in the field, and thus permit 



Abbe's 

 Test Platte 



C.Zeiss 

 Jena 



Fig. 34. 



maker, a diaphragm pierced with say 

 three openings of f inch diameter, in 

 which circles of card may be dropped, 

 the card being pierced with holes of 

 different sizes according to the direc- 

 tions given above. 



Another plan adopted by Dr. Fripp 

 and found very convenient in practice 

 is to mount a condensing lens (Profes- 

 sor 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 



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 strips, 

 opaque and clear lines alternating at 

 close intervals, and lying absolutely 

 in the same plane, so that no devia- 

 tion 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 in- 

 finite thinness, and fixed by known 

 methods on glass. Cover-glasses of 

 various thickness, from 0.24 mm. to 

 0.09 mm. (accurately measured), are 

 ruled on one surface thus coated with 

 a film of metal, the groups of lines 

 varying from aio to ^t^ inch ; the ruled 



