44 LIGHTING AND FOCUSING. \_CH. II. 



§ 80. Aperture of the Illuminating Cone and the Field. — It is 



to be remarked that with a very small source of light the entire aper- 

 ture of the objective may be filled if a proper illuminator or condenser 

 is used. The aperture depends on the diaphragm used with the con- 

 denser. And the size of the diaphragm must be directly as the aper- 

 ture of the objective. That is, it is just the reverse of the ride for 

 diaphragms where no condenser is used (§ 63) ; for there the diaphragm 

 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 

 partly 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 ac- 

 complished 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). 



§ 81. Non- Achromatic Condenser. — Of the non-achromatic con- 

 densers or illuminators, the Abbe condenser or illuminator is the one 

 most generally used. It is also much more commonly used than the 

 achromatic condenser from its cheapness. 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 trans- 

 mitted 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 con- 

 denser like the Abbe are much more satisfactory than with no condenser. 

 The highest results cannot be attained with it, however. (Carpenter- 

 Dallinger, p. 256). 



