54 LIGHTING AND FOCI 'SIXG [ CH. II 



the condenser so that the object is not in the focus of the condenser, 

 but above or below it, and therefore lighted by a converging or 

 diverging beam where the light is spread over a greater area (Figs. 

 54-57. 99)- 



96. Non-Achromatic Condensers. Of the non-achromatic 

 condensers or illuminators, the Abbe condenser or illuminator is the 

 one most generally used. From its cheapness it is also much more 

 commonly used than the achromatic condenser. It consists of two 

 or three very large lenses and transmits a cone of light of 1.20 N.A. 

 to 1.40 N.A., Figs. 58-59, but the aberrations, both spherical and 

 chromatic, are very great in both forms. Indeed, so great are they 

 that in the best form with three lenses and an illuminating cone of i . 40 

 N. A., the aplanatic cone transmitted is only 0.5, and it is the apla- 

 naticcone which is of real use in microscopic illumination where de- 

 tails are to be studied. There is no doubt, however, that the results 

 obtained with a non-achromatic condenser like the Abbe are much 

 more satisfactory than with no condenser. The highest results can- 

 not be attained with it, however. ( Carpenter- Dallinger, p. 309.) 



97. Position of the Condenser. The proper position of 

 the illuminator for high objectives is one in which the beam of light 

 traversing it is brought to a focus on the object. If parallel rays 

 are reflected from the plane mirror to it, they will be focused only a 

 few millimeters above the upper lens of the condenser ; consequently 

 the illuminator should be about on the level of the top of the stage 

 and therefore almost in contact with the lower surface of the slide. 

 For some purposes when it is desirable to avoid the loss of light by 

 reflection or refraction, a drop of water or homogenous immersion 

 fluid is put between the slide and condenser, forming the so-called 

 immersion illuminator. This is necessary only with objectives of 

 high power and large aperture or for dark-ground illumination. 



98. Centering the Condenser. The illuminator should be 

 centered to the optic axis of the microscope, that is the optic axis 

 of the condenser and of the microscope should coincide. Unfortun- 

 ately there is extreme difficulty in determining when the Abbe 

 illuminator is centered. Centering is approximated as follows : 

 Put a pin-hole diaphragm that is a diaphragm with a small central 

 hole over the end of the condenser (Fig. 58), the central opening 

 should appear to be in the middle of the field of the microscope. If 



