308 ACCESSORY APPARATUS 



close the diaphragm so that the image of it just appears at the back 

 of the objective, we are able to determine the aperture of the illumi- 

 nating cone with that given opening in the diaphragm ; thus in fig. 

 251 it is a trifle less than '5 N.A. 



In a similar manner the apertures of the other diaphragm open- 

 ings can be determined. 



Now let the diaphragm be opened to the full aperture, and an 

 objective with a wider aperture, say "95, be used. It will perhaps 

 be found that before we are able to fill the back of the objective with 

 light by racking up the condenser, two black spots will be formed on 

 either side the middle of the disc. When we reach the disc of light 

 that is largest (fig. 252), any further racking up causes the appear- 

 ance shown in fig. 253. The last point before the appearance of the 

 black spots indicates the largest aplanatic aperture of the condenser, and 

 is the limit of the condenser for critical work}- 



There are many other condensers of more or less merit and use- 

 fulness than those which we have already described and illustrated ; 

 but for most recent lenses, and for the finest critical results, we have 

 given them as full a representation as can be fairly desired. But 

 there are still some forms that either from their own peculiar value 

 or their historic importance deserve consideration. 



A condenser known as the * Webster ' was first made in 1865, and 

 is still a very useful one for low powers. It is the same as that 

 made by Swift, but without the middle combination. Its angle is 

 less, and its range is not so extensive ; but its chief commendation in 

 possessing these qualities is that, having one combination less than 

 Swift's, it is of necessity lower in price, and on that account will be 

 welcome to some workers. 



In its present form it reverses its primary construction. It is 

 now made with a double front and a single back, instead of a single 

 front and a double back. 



A chromatic condenser which has been very largely used in 

 England and America, and which has secured a great deal of com- 

 mendation, is that of Professor Abbe. The optical productions of 

 Abbe are too well known and too valuable as a rule to make it 

 needful to be other than perfectly frank concerning so important a 

 piece of apparatus as this ; and there can be no doubt that the wide 

 popularity of this instrument is due, not so much to intrinsic merit 

 as to the fact that it has been employed much by those who, 

 previously ignorant of the value of any .condenser, have at once 

 perceived the enhanced value of the results yielded by its means. 



To those who have made the scientific use of the microscope a 

 careful study in England it has been a persistent source of regret 

 that it was so long and pertinaciously taught that the * correct ' 

 histological microscope must be of the Hartnack type, and that it 

 should be used with narrow-angled dry lenses, perhaps a j-th-inch 

 focus, and no illumination but that afforded by a small concave 

 mirror, the focal point of which is extremely doubtful or unknown, 



1 ' The Back of the Objective and the Condenser.' E. M. Nelson, Eng. Mech. 

 vol. xlviii. No. 1234, 1883. 



