CH. II] DARK-GROUND ILLUMINATION WITH HIGH POWERS 73 



plish this a reducing diaphragm is introduced into the objective cutting down the 

 aperture to less than i.oo N.A. (fig. 49). It would be a convenience if homo- 

 geneous immersion objectives were made with an aperture less than i.oo N.A. to use 

 in dark-field work. 



125b. Aperture of reducing diaphragm. While the reducing diaphragm 

 for the objectives with an aperture above i.oo N.A. must be of a size to reduce 

 the aperture to less than i.oo N.A. in choosing the size it 

 must be remembered that in dark-field as ^in bright-field 

 microscopy the resolution is^ directly as the aperture, and 

 the brilliancy with the square of the aperture. Therefore 

 the upper limit of the aperture of the objective should be 

 as great as possible without endangering the loss of the 

 dark-field effect. As a result of careful experiments made 

 with reducing diaphragms from 0.40 to 0.97 N.A. ^dia- 

 phragms the conclusion was reached that if but one 

 reducing diaphragm is available, one giving an aperture of 

 0.80 N.A. is the most generally useful, and gives a dark- 

 field with practically all the dark-field condensers one is 

 likely to meet. If much work on a great variety of ob- 

 jects and with different sources of light is to be done 

 it would be a great advantage to have reducing dia- 

 phragms of 0.70, 0.80 and 0.90 N.A., then the one which 

 gave the best effect in a given case could be used. These 

 diaphragms are easily introduced into the objective and as 

 easily removed. One should bear in -mind that the low- 

 er end of the diaphragm must be very close to the back lens 

 of the objective. It is also to be remembered that when 

 the objective is to be used for the bright-field microscopy 

 the diaphragm should be removed so that the full benefit 

 of the aperture is secured. ' 



FIG. 49. HIGH- 

 POWER OBJECTIVE 

 WITH APERTURE RE- 

 DUCING DIAPHRAGM 

 FOR DARK-GROUND 

 ILLUMINATION. 



(From Chamot.) 



D Funnel-shaped 

 reducing diaphragm 

 screwed into the lower 

 end of the "boot" 

 opposite (R). 



126. Immersion connection of condenser 

 and glass slide. While the purpose of the 

 reflecting condenser is to produce a very 

 oblique beam of light for illuminating the objects, it is evident at 

 once that the laws of refraction will prevent the light above a certain 

 angle from passing out of the condenser to the object unless the glass 

 slide is in immersion contact with the condenser. Figure 50 shows 

 the aperture of the light which can pass from the condenser when 

 air, water, glycerin or homogeneous liquid is above it. 



Reasons for always using homogeneous immersion. While it is true 

 that the medium of least refractive index determines the angle of light that can 

 pass from a denser to a rarer medium (fig. 50), it is also true that objects sur- 

 rounded by air, water, etc., may be in optical contact with the glass slide, in which 

 case the light will pass into the object up to the limit of the refractive index of the 

 object, hence the desirability of the light beam being of great obliquity, even though 

 the object is mounted in air. One can demonstrate this by cleaning the upper 

 surface of the condenser with the greatest k care, noting that it is very dark 



