WRIGHT: MEASUREMENT OF REFRACTIVE INDICES 103 



located in the lower focal plane of the condenser and imaged 

 in the eye-circle of the ocular. The angle ordinarily employed 

 with a 16 mm. objective (Zeiss apochromatic ; N. A. = 0.30) is 

 8° to 10°; under these conditions the diameter of the eye-circle 

 (compensating eyepiece 4) is 1.3 mm. This angle can be reduced 

 to 4° by closing the iris diaphragm as far as possible; but the 

 diameter of the eye-circle is then only 0.6 mm., the illumina- 

 tion of the field is weak and dust particles are troublesome. 

 In a high power objective (Zeiss apochromatic, E. F. = 4 mm.; 

 N. A. = 0.95) the angular aperture usually employed in central 

 illumination is about 15°; for this aperture the diameter of the 

 eye-circle is 0.5 mm. In case the smallest possible aperture, 

 4°, be used, the diameter of the eye-circle is reduced to 0.15 

 mm.; the field illumination is then so weak that a strong light 

 source is necessary; all lenses and also object cover glass must 

 be thoroughly cleaned to reduce, as far as possible, the dis- 

 turbing effects resulting from dust particles and from dif- 

 fraction. In oblique illumination a narrow pencil of obliquely 

 incident light is employed and is best obtained by means either 

 of a movable iris diaphragm or of a sliding metal strip in the 

 lower focal plane of the substage condenser. Experience has 

 shown that in the case of the sliding substage-stop the best 

 effects are produced when the stop is inserted past the axis to 

 a point midway between the center and the margin of the con- 

 denser opening. Under these conditions the inclination of the 

 incident rays in the 16 mm. objective ranges from 9° to 18°, 

 with an average inclination of 12.5°. This angle can be de- 

 creased by closing the substage iris diaphragm and withdrawing 

 the sliding stop a short distance; but then the intensity of 

 field illumination decreases rapidly and the disturbing effects 

 of dust particles become serious. With a 32 mm. objective 

 (N. A. = 0.10) the angular field measures 6° on a side and the 

 best results in oblique illumination are obtained for rays of 

 an average angular inclination of 4° (3° to 6°). With the 4 

 mm. objective it is difficult to obtain proper illumination by 

 this method because of the rapid decrease in definition of image 

 with oblique, one sided illumination. 



