424 MANIPULATION AND PRESEKVATION OF THE MICROSCOPE 



tion with a dark ground. The best that can be done is to use an 

 oil-immersion condenser with a suitable stop ; this will give a good 

 dark ground up to '65 N.A., but it will fail if the object is dry on 

 the cover. Generally speaking, the only way of accomplishing this 

 with objectives of wider aperture is to reduce the aperture of the 

 objective by a stop placed at the back. 



When a condenser is united by a film of oil to a slip, if the slip 

 is thin, the oil invariably runs down when the condenser is focussed. 



The following is a method 

 by which this may be en- 

 tirely prevented. A piece 

 of thick cover-glass about 

 02 inch, and 1 inch square, 

 has a strip of thicker glass, 

 jj- inch broad, cemented by 

 shellac to one edge. This 

 piece of glass is oiled to 



Thin slip of glass with the slip, the ledge being' 



ledge to place glass hooked over the top of the 



slip with oil contact, Slide in situ on thin ,., , , . 



so as to vary the slip with ledge. slide; this not Only pre- 



thickness of a slide. vents its slipping down, 



FIG. 3(i5. but also keeps the oil from 



creeping out at the bottom, 



which would be the case if the two edges of the glass coincided. 1 

 This is illustrated in fig. 365. 



In its proper place we have dealt with the suitable relation of 

 aperture to power, and have pointed out the irresistible nature of 

 the contentions and teachings of Abbe on the subject. Here a 

 direct practical presentation of the matter may be of service to the 

 student. 



A normal unaided human eye can divide ^\-^ inch at ten inches. 

 Consequently a microscope with a power of 200 should be capable of 

 showing structure as fine as S^OTT inch. Xow, as this power can be 

 made up by ^-inch objective and a 1-inch eye-piece, it follows that 

 sufficient aperture ought to be given to the i-inch to enable it to 

 resolve 50,000 lines per inch. This 2 will be '52 N.A. The inch 

 objective should have half this aperture, and the J double, and the 

 ^ four times as much, if perfect vision is required ; in other words, 

 26 N.A. for every 100 diameters. 3 These ideals have (as we have 

 before indicated) been realised, notably by the Zeiss apochromatics, 

 the 1-iiich and the ^-inch 4 resolving everything capable of being 

 appreciated by the eye when the 12 compensating eye-piece is used. 

 The J-inch is also a near approach to the ideal, as it has been very 

 wisely kept a dry lens. The oil-immersion J-in. of 1*4 N. A. with 

 a 6 eye-piece also attains the ideal. This relation of aperture to 



1 Q. M. C. Jouriial, November 1885. 



2 In reality it will require more, because an axial cone is assumed to be used 

 instead of an oblique beam. 



3 English Mechanic, vol. xxxviii. 1888, No. 979. E. M. Nelson. 



4 This lens, with an 8 compensating eye-piece, will resolve a Pleurosigma 

 angulatum with an axial cone ; this is the lowest power with which it has ever been 

 done. 



