CH. II] LIGHTING AND FOCUSING 41 



it (see Ch. III). Use a 3 mm.,(^ in.) or No. 7 objective and a medium 

 ocular. Focus the microscope and select a very small bubble, one 

 whose image appears about 1 mm. in diameter, then arrange the plane 

 mirror so that the light spot in the bubble appears exactly in the 

 center. Without changing the position of the mirror in the least, 

 replace the air bubble preparation by one of Pleurosigma angulatum or 

 some other finely marked diatom. Study the appearance very carefully. 



§ 78. Oblique Light (§ 66). — Swing the mirror far to one side 

 so that the rays reaching the object may be very oblique to the optic 

 axis of the microscope. Study carefully the appearance of the diatom 

 with the oblique light. Compare the appearance with that where central 

 light is used. The effect of oblique light is not so striking with histo- 

 logical preparations as with diatoms. 



It should be especially noted in §§ 77, 78, that one cannot deter- 

 mine the exact direction of the rays by the position of the mirror. 

 This is especially true for axial light ( §77). To be certain the light 

 is axial some such test as that given in § 77 should be applied. (See 

 also Ch. Ill, under Air-bubbles.) 



CONDENSERS OR ILLUMINATORS* 



§ 79. These are lenses or lens-systems for the purpose of illuminat- 

 ing 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. 



*No one has stated more cleaily, or appreciated more truly the value of cor- 

 rect illumination and the methods of obtaining it than Sir David Brewster, 1820, 

 1831. 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 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. 



