58 LIGHTING AND FOCUSING [CH. II 



When one is through using a water immersion objective, remove it 

 from the microscope and with some lens paper wipe all the water from 

 the front lens. Unless this is done dust collects and sooner or later 

 the front lens will be clouded. It is better to use distilled water to 

 avoid the gritty substances that are liable to be present in natural 

 waters, as these gritty particles might scratch the front lens. 



HOMOGENEOUS IMMERSION OBJECTIVES : EXPERIMENTS 



§ 105. As stated above, these are objectives in which a liquid of 

 the same refractive index as the front lens of the objective is placed 

 between the front lens and the cover-glass. 



§ 106. Tester for Homogeneous Liquid. — In order that full 

 advantage be derived from the homogeneous immersion principle, the 

 liquid employed must be truly homogeneous. To be sure that such is 

 the case, one may use a tester like that constructed by the Gundlach 

 Optical Co. , then if the liquid is too dense it may be properly diluted 

 and vice versa. For the cedar oil immersion liquid, the density may 

 be diminished by the addition of pure cedar wood oil. The density 

 may be increased by allowing it to thicken by evaporation. (See H. 

 L. Smith, Proc. Amer. Soc. Micr., 18S5, p. S3, and Ch. X). 



§107. Refraction Images. — Put a 2 mm. ( T l T th in.) homogeneous 

 immersion objective in position, employ an illuminator. Use some 

 histological specimen like a muscular fiber as object, make the dia- 

 phragm opening about 3 mm. in diameter, add a drop of the homo- 

 geneous immersion liquid and focus as directed in § 74. The object 

 will be clearly seen in all details by the unequal refraction of the light 

 traversing it. The difference in color between it and the surrounding 

 medium will also increase the sharpness of the outline. If an air bub- 

 ble preparation (§ 77) were used, one would get pure, refraction 

 images. 



§ 108. Color Images. — Use some stained bacteria as Bacillus 

 tuberculosis for object. Put a drop of the immersion liquid on the 

 cover-glass or the front lens of the homogeneous objective. Remove 

 the diaphragms from the illuminator or in case the iris diaphragm is 

 used, open to its greatest extent. Focus the objective down so that 

 the immersion fluid is in contact with both the front lens and the cover- 

 glass, then with the fine adjustment get the bacteria in focus. They 

 will stand out as clearly defined colored objects on a bright field. 



