710 



SUMMARY OF CUKRENT RESEARCHES RELATING TO 



apochromat 2 mm. X.A. 1 "30, with intense dark-field stop. This dark- 

 field stop is after Prof. Abbe's suggestion, and has the front lens of the 

 objective in its central part as far as aperture 0'33 mm. accurately cut 

 away, and the corresponding plane face blackened. The only rays 

 which enter the objective are those between apertures 0*3 and 1*8. 



Among the advantages of the arrangement are : Firstly, absence of 

 reflexions between the lenses ; secondly, the tedious centring for dark- 

 ground illumination is obviated ; thirdly, a stop made like this cannot 

 be decentred ; and lastly, the objective remains available also for ob- 

 servation in the ordinary way without dark-ground illumination. If 

 the illumination should now appear unequal, the inequality is due to 



Fig. 116. 



the fact that the reduced image of the light source of perhaps 0*1 mm. 

 does not generally lie in the field of view of the observation objective. 

 To correct this, two centring screws, e lt c 2 (of which only one is visible 

 in figs. 116 and 117), are provided, and must be so adjusted that the 

 image of the light source appears in the preparation. This image is 

 only indirectly visible, as only the particles which are encountered by 

 the beams appear self-luminous through diffraction. The centring 

 succeeds best with a not too high ocular (perhaps compensation ocular 

 No. 4). The illumination can be increased or reduced by rotation of 

 the ring r. 



In the use of liquid films between object-carrier and cover-glass, 

 care must be taken that the film thickness is neither too great nor too 



