CH, II] LIGHTING AND FOCUSING. 39 



til the front of the objective just touches the cover-glass. Note the po- 

 sition of the micrometer screw and slowly focus up with the fine ad- 

 justment until the object is in focus. The distance the objective was 

 raised plus the thickness of the cover-glass represents the working dis- 

 tance. For example, a 3 mm. objective after being brought in contact 

 with a cover-glass was raised by the fine adjustment a distance repre- 

 sented by 16 of the divisions on the head of the micrometer screw. 

 Each division represented .01 mm., consequently the objective was 

 raised .16 mm. As the cover-glass on the specimen used was .15 mm. 

 the total working distance is .16 + .15 = .31 mm. 



CENTRAL AND OBLIQUE LIGHT WITH A MIRROR. 



§ 73. Axial or Central Light (§ 61). — Remove the condenser or 

 any diaphragm from the substage, then place a preparation containing 

 minute air bubbles under the microscope. The preparation may be 

 easily made by beating a drop of mucilage on a slide and covering it. 

 (See Ch. III). Use a 3 mm. (}i in.) or No. 7 objective and a medi- 

 um 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 Pleurasigvia angulatum or some other 

 finely marked diatom. Study the appearance very carefully. 



§ 74. Oblique Light, (§ 62). — 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 different appearance with that of cen- 

 tral light. The effect of oblique light is not so striking with histologi- 

 cal preparations as with diatoms. 



It should be especially noted in §§ 73, 74, that one cannot deter- 

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

 is especially true for axial light (§ 73). To be certain that the light is 

 axial some such test as that given in § 73 should be applied. (See 

 also Ch. Ill, under Air-bubbles). 



CONDENSERS OR ILLUMINATORS.* 



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

 with transmitted light the object to be studied with the microscope. 



*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. 



