46 LIGHTING AND FOCUSING [CH. II 



is made large for low powers, and consequently low apertures, while 

 with the condenser the diaphragm is made small for low and large for 

 high powers as the aperture is greater in the high powers of a given 

 series of objectives. It is very instructive to demonstrate this by using 

 a 16 mm. objective and opening the diaphragm of the condenser till the 

 back lens is just filled with light. Then if one uses a 3 or 4 mm. ob- 

 jective it will be seen that the back lens of the higher objective is only 

 partty filled with light and to fill it the diaphragm must be much more 

 widely opened. 



With a condenser, then, the diaphragm has simply to regulate the 

 aperture of the illuminating cone, and has nothing to do with lighting 

 a large or a small field. 



With the condenser there are two conditions that must be fulfilled, 

 — the proper aperture must be used, and that is determined by the dia- 

 phragm, and secondly the whole field must be lighted. The latter is 

 accomplished by using a larger source of light, as the face instead of 

 the edge of a lamp flame, or by lowering or raising 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. 48-51, § 88). 



§ 85. Non- Achromatic Condenser. — 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 com- 

 monly 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., but the aberrations, both spherical and chromatic, are very great in 

 both forms. Indeed, so great are they that in the best form of three 

 lenses with an illuminating cone of 1.40 N. A., the aplanatic cone 

 transmitted is only 0.5, and it is the aplanatic cone which is of real use 

 in microscopic illumination where details 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 con- 

 denser. The highest results cannot be attained with it, however. 

 ( Carpenter- Dallinger, p. 309). 



§ 86. Arrangement 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 



