CH. II.-] LIGHTING AND FOCUSING. 55 



engraving on each objective the " tube-length " for which it is correct- 

 ed, is to be commended, for it is manifestly difficult for each worker 

 with the microscope to find out for himself for what ' ' tube-length ' ' 

 each of his objectives was corrected. (See appendix). 



§ 97. Water Immersion Objectives. — Put a water immersion ob- 

 jective in position (§ 43) and the fly's wing for object under the micro- 

 scope. Place a drop of distilled water on the cover-glass, and with the 

 coarse adjustment lower the tube till the objective dips into the water, 

 then light the field well and turn the fine adjustment one way and the 

 other till the image is clear. Water immersions are exceedingly con- 

 venient in studying the circulation of the blood, and for many other 

 purposes where aqueous liquids are liable to get on the cover-glass. If 

 the objective is adjustable, follow the directions given in § 95. 



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 would 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. 



§ 98. 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. 



§ 99. Tester for Homogeneous Liquid. — In order that full ad- 

 vantage 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., 1885, p. 83, and appendix). 



§ 100. Refraction Images. — Put a 2 mm. (xV^ 1 i' 1 -) 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 § 70. 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 



