4 8 LIGHTING AND FOCUSING [ CH. II 



As the cover- glass on the specimen used was 0.15 mm. the total 

 working distance is 0.16+0.15=0.31 mm. 



CENTRAL AND OBLIQUE LIGHT WITH A MIRROR 



§ 88. Axial or Central Light (§ 74). — Remove the con- 

 denser or any diaphragm from the substage, then place a preparation 

 containing minute air bubbles under the microscope. The prepara- 

 tion may be easily made by beating a drop of mucilage on the slide 

 and covering it (see Ch. III). Use a 4 or 3 mm., (>fj.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. 



§ 89. Oblique Light {§ 75). — 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 histological preparations as with diatoms. 



It should be especially noted in §§ 88, 89, that one cannot 

 determine the exact direction of the rays by the position of the mir- 

 ror. This is especially true for axial light (§88). To be certain 

 the light is axial some such test as that given in § 88 should be 

 applied. (See also Ch. Ill, under Air-bubbles.) 



CONDENSERS OR ILLUMINATORS* 



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



*No one has stated more clearly, or appreciated more truly the value of 

 correct 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 con- 



