40 LIGHTING AND FOCUSING. • [CH. II 



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

 mated. 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 with rack and pinion 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. of the condenser utilized with different openings. Finally, 

 the condenser should be supplied with central stops for dark-ground 

 illumination (§88) and with blue and neutral tint glasses to soften the 

 glare when artificial light is used (§ 85, 89). 



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. 



§ 76. Achromatic Condenser. — It is still believed by all expert mi- 

 croscopists that the contention of Brewster was right, and the condenser 

 to give the greatest aid in elucidating microscopic structure must ap- 

 proach in excellence the best objectives. That is, it should be as free as 

 possible from spherical and chromatic aberration, and therefore would 

 transmit to the object a very large aplanatic cone of light. Such con- 

 densers 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 achromatic condensers should be used for all 

 work. Unfortunately good condensers like good objectives are expen- 

 sive, and student microscopes as well as many others are mostly supplied 

 with the non-achromatic condensers or with none. 



Many excellent achromatic condensers have been made, but the most 

 perfect of all seems to be the apochromatic of Powell and Zealand (Car- 

 He says of illumination in general : " The art of illuminating microscopic objects 

 is not of less importance than that of preparing them for observation." "The eye 

 should be protected from all extraneous light, and should not receive any of the 

 light which proceeds from the illuminating center, excepting that portion of it 

 which is transmitted through or reflected from the object." So likewise the value 

 and character of the substage condenser was thoroughly understood and pointed 

 out by him as follows : "I have no hesitation in saying that the apparatus for illu- 

 mination 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 chro- 

 matic and spherical aberration, and the greatest care be taken to exclude all ex- 

 traneous 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 (1S31), p. 83. 



