CH. III.] INTERPRETATION OF APPEARANCES. 85 



pear in the center as with air. Focus up and down as with air, and note 

 that the bright center of the oil globule is clearest last in focusing up. 



Fig. 88. Very small Globule of Oil (O) and an Air Bubble (A) A 



seen by Oblique light. The arrow indicates the direction of the 

 light rays. 



§ 132. Oil Globules with Oblique Illumination. — Re- 

 move the sub-stage, etc., as above, and swing the mirror to 

 one side and light with oblique light. The bright spot will 

 be eccentric, and will appear to be on the same side as the 

 mirror (Fig. 88). 



§ 133. Oil and Air Together. — Make a preparation ex- 

 actly as described for air bubbles (§ 127), and add at one 

 edge a little of the mixture of oil and mucilage (§ 130) ; 

 cover and examine. 



The sub-stage need not be used in this experiment. Search the prep- 

 aration until an air bubble and an oil globule, each appearing about 1 

 mm. in diameter, are found in the same field of view. Light first with 

 central light, and note that, in focusing up, the air bubble comes into 

 focus first and that the central spot is smaller than that of the oil globule. 

 Then, of course, the black ring will be wider in the air bubble than in 

 the oil globule. Make the light oblique. The bright spot in the air 

 bubble will move away from the mirror while that in the oil globule will 

 move toward it. See Fig. 88.* 



§ 134. Air and Oil by Reflected Light. — Cover the diaphragm or 

 mirror so that no transmitted light (§ 60} can reach the preparation, 

 using the same preparation as in § 133. The oil and air will appear 

 like globules of silver on a dark ground. The part that was darkest in 

 each will be lightest, and the bright central spot will be somewhat 



dark.y 



§135. Distinctness of Outline. — In refraction images this depends 

 on the difference between the refractive power of a body and that of the 

 medium which surrounds it. The oil and air were very distinct in out- 



* It should be remembered that the image in the compound microscope is in- 

 verted (Fig. 21), hence the bright spot really moves toward the mirror for air, and 

 away from it for oil. 



f It is possible to distinguish oil and air optically, as described above, only when 

 quite high powers are used and very small bubbles are selected for observation. If 

 a 16 mm. (%in.) is used instead of a 3 mm. (}$ in.) objective, the appearances 

 will vary considerably from that given above for the higher power. It is well to 

 use a low as well as a high power. Marked differences will also be seen in the ap- 

 pearances with objectives of small and of large aperture. 



