Ch. II] LIGHTING WITH A SUBSTAGE CONDENSER 57 



Condensers or Illuminators 



§ 100. Condensers. — These are lenses or lens systems for the 

 purpose of illuminating with transmitted Ught the object to be studied 

 with the microscope (§ looa). 



For the highest kind of investigation their value cannot be over- 

 estimated. They may be used either with natural or artificial light, 

 and should be of sufiicient numerical aperture (N.A.) to satisfy the 

 widest angle objectives to be used. 



It is of great advantage to have the substage condenser mounted 

 so that it may be moved up and down under the stage. An iris 

 diaphragm is now almost xmiversally employed, and with some there 

 is a scale showing the numerical aperture (N.A.) of the cone of light 

 given in each position of the iris. Finally it is an advantage to have 

 a stop holder and diaphragms with central stops under the condenser 

 for the production of dark-ground illmnination (§ 122). 



Condensers or illuminators fall into two great groups, the achro- 

 matic, giving a large aplanatic cone, and non-achromatic, giving 

 much light, but a relatively small aplanatic cone of light. 



§ 100a. No one has stated more clearly, or appreciated more truly the 

 value of correct illumination arid the methods of obtaining it than Sir David 

 Brewster, 1820, 1831. He says of illumination in general: "The art of illu- 

 minating microscopic objects is not of less importance than that of preparing 

 them for observation." "The eye should be protected from all extraneous 

 Ught, and should not receive any of the light which proceeds from the illuminat- 

 ing center, excepting that portion of it which is transmitted through or reflected 

 from the object." So likewise the value and character of the substage con- 

 denser was thoroughly understood and pointed out by him as follows: "I 

 have no hesitation in saying that the apparatus for illumination requires to 

 be as perfect as the apparatus for vision, and on this account I would recom- 

 mend that the illuminating lens should be perfectly free of chromatic and 

 spherical aberration, and the greatest care be taken to exclude all extraneous 

 light both from the object and from the eye of the observer." See Sir David 

 Brewster's treatise on the Microscope, 1837, pp. 136, 138, 146, and the Edin- 

 burgh Journal of Science, new series, No. 11 (1831) p. 83. 



§ 101. Achromatic condenser. — It is still beUeved by all expert 

 microscopists that the contention of Brewster was right, and the 

 condenser to give the greatest aid in elucidating microscopic structure 

 must approach in excellence the best objectives. That is, it should 

 be as free as possible from spherical and chromatic aberration, and 



