24 MICROSCOPE AND ACCESSORIES. 



• 



the standard it should'be remembered that : (A) If the tube of the mi- 

 croscope is longer than the standard for which the objective was cor- 

 rected, the effect is approximately the same as thickening the cover- 

 glass, and therefore the systems of the objective must be brought closer 

 together, i. e., the adjusting collar must be turned away from the zero 

 mark. (B) If the tube is shorter than the standard for which the ob- 

 jective is corrected, the effect is approximate^ the same as diminishing 

 the thickness of the cover-glass, and the systems must therefore be sep- 

 arated, that is, the adjusting collar turned tozvard the zero mark (Fig. 



57)- 



Furthermore, whatever the interpretation by different opticians of 

 what should be included in "tube-length," and the exact length in 

 millimeters, its importance is very great ; for each objective gives the 

 most perfect image of which it is capable with the ' ' tube-length ' ' for 

 which it is corrected, and the more perfect the objective the greater the 

 ill effects on the image of varying the "tube-length " from this stand- 

 ard. The plan of designating exactly what is meant by ' ' tube-length, ' ' 

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

 rected, 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. 



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

 jective in position (§ 30) 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 § 63. 



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

 from the microscope and with some Japanese bibulous paper (§ 72) 

 wipe all the water from the front-lens. Unless this is done dust col- 

 lects 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. 



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

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

 tween, the front-lens and the cover glass. 



