CH. II] LIGHTING AND FOCUSING 49 



illuminating with transmitted light the object to be studied with 

 the microscope. 



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 sufficient numerical aperture to satisfy 

 objectives of the widest angle. 



It is of the greatest advantage to have the sub-stage condenser 

 mounted so that it may be easily moved up or down under the stage. 

 The iris diaphragm is so convenient that it should be furnished in 

 all cases, and there should be marks indicating the N. A. (§36) of the 

 condenser utilized with different openings. Finally the condenser 

 should be supplied with central stops for dark-ground illumination 

 (§ io 3) an d with blue and neutral tint glasses to soften the glare 

 when artificial light is used (§ 100, 104). 



Condensers or Illuminators fall into two great groups, the 

 Achromatic, giving a large aplanatic cone, and Non-achromatic, 

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



§ 91. Achromatic Condenser. — It is still believed by all ex- 

 pert 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 aberra- 

 tion, and therefore would transmit to the object a very large aplan- 

 atic cone of light. Such condensers are especially recommended 

 for photo-micrography by all, and those who believe in getting the 

 best possible image in every case are equally strenuous that achro- 

 matic condensers should be used for all work. Unfortunately good 

 condensers like good objectives are expensive, and student micro- 

 scopes as well as many others are usually supplied with the non- 

 achromatic condensers or with none. 



Many excellent achromatic condensers have been made, but the 



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 recommend 

 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 Edinburgh Journal 

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



