300 TESTING HOMOGENEOUS LIQUID [Ch. IX 



and depth to receive the tester which is just above the objective. As 

 object it is well to employ a stage micrometer and to measure carefuUy 

 the diameter of the field without the tester, then with the tester far 

 enough inserted to permit of the passage of rays through the glass 

 but not through the concavity, and finally the concavity is brought 

 directly over the back lens of the objective. This can be easily deter- 

 mined by removing the ocular and looking down the tube. 



Following Professor Smith's directions it is a good plan to mark in 

 some way the exact position of the tube of the microscope when the 

 micrometer is in focus without the tester, then with the tester pushed 

 in just far enough to allow the Hght to pass through the plane glass, 

 and finally when the light traverses the concavity. The size of the 

 field should be noted also in the three conditions (§ 47-49). 



It is seen by glancing at the following table that whenever the 

 hquid in the tester is of lower index than glass, the concavity with the 

 hquid acts as a concave lens, or in other words like an amplifier 

 (§ 236a), and the field is smaller than when no tester is used. It is 

 also seen that as the Hquid in the concavity approaches the glass in 

 refractive index, the field approaches the size when no tester is 

 present. It is also plainly shown by the table that the greater the 

 difference in refractive index of the substance in the concavity and 

 the glass, the more must the tube of the microscope be raised to 

 restore the focus. 



If a substance of greater refraction than glass is used in the tester 

 the field is larger, i. e., the magnification less, and one would have to 

 turn the tube down instead of up to restore the focus. 



The table given below indicates the changes when using a tester 

 prepared by the Gundlach Optical Co., and used with a 16 mm. 

 apochromatic objective of Zeiss, X 4 compensation ocular, achro- 

 matic condenser, i.oo N. A. (fig. 40): 



§ 474. Diffracted light in microscopy. — As most microscopic ob- 

 servation depends upon directed light from some source Uke the sun 

 or a lamp sent to and through the object by a mirror only or by the 

 aid of a condenser or a mirror and condenser, the phenomena of dif- 

 fraction are present. It is evident that if the objects observed were 

 seK-luminous the conditions would be different from those existing 



