8 ON OPTieAL aUALITY OF MICROSCOPE OBJECTIVES. 



2. The means ly which such isolated pencils^can he obtained. 



If a special illuminating apparatus be employed^ the condenser 

 of Professor Abbe will be found very convenient — but almost any 

 condenser of the kind (hemispherical lens) e.g., Collins' or 

 Reade's may be arranged for this purpose. 



In the lower focal plane of the illuminating lens must be fitted 

 diaphragms (easily made of blackened cardboard) pierced with two 

 or three openings of such a size that their images, as formed by 

 the objective, may occupy a fourth or sixth part of the diameter of 

 the whole aperture (i.e., of the field seen when looking down the 

 tube of the instrument, after removing the ocular, on the objective 

 image.) The required size of these holes, which depends, firstly, 

 on focal length of the illuminating lens and, secondly, on the angle 

 of aperture of the objective, may be thus found. A test object 

 being first sharply focussed, card diaphragms having holes of 

 various sizes (two or three of the same size in each card) must be 

 tried until one size is found, the image of which in the posterior 

 focal plane of the objective shall be about a fourth to a sixth part 

 of the diameter of the field of the objective. Holes having 

 the dimensions thus experimentally found to give the required 

 size of image must then be pierced in a card, m such 

 position as will produce images situate in the field as shown by 

 figure I and 2, and the card is then fixed in its place below the 

 condenser. If the c ondenser be fitted so as to 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 condenser 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 beside 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 f inch diameter in 

 which circles of card may be dropped 3 the cards being pierced with 

 holes of different sizes according to the directions given above. 



