THE WATER-IMMERSION OBJECTIVE 133 



marked difference, and large cover-glasses may vary to this 

 degree or more in thickness in different parts. It is best, 

 even with an objective having a correction collar, to select 

 covers with a narrow range of thickness; about 0.14 to 0.17 

 millimeter, where 0.17 is the standard; for, in the writer's 

 experience, there is a more perfect image when the cover 

 thickness is equal to the standard, even when the correction 

 collar is used with skill. The best way to use the correction 

 collar is to measure the cover with a gage; or, when the 

 cover is already on a slide, to calculate its thickness by 

 focusing with a dry lens, from the lower to the upper surface 

 of the cover (or vice versa), and multiplying the reading of 

 the micrometer by 1.5. (With the new fine motion of 

 Zeiss, where each scale division is 2 microns, the reading 

 is simply multiplied by 3.) The finer correction is then 

 made by observation of the object as the collar is moved 

 shghtly each way. The true correction may be marked 

 on the cover with India ink. One optical firm makes a 

 water-immersion objective of aperture 1.2, without a collar. 

 The writer finds this is best used by slightly unscrewing the 

 two front lenses, for covers below 0.17, instead of correcting 

 by lengthening the drawtube. It cannot be used with 

 thick covers. 



Necessary Precautions. — To get perfect results, certain 

 precautions must be taken with a water-immersion 

 objective: 



1. Distilled water, and not tap water, should be used for 

 immersion, since otherwise the salts in the tap water may in 

 time form a deposit on the front lens. (Even in a small 

 bottle of distilled water, bacteria will often multiply, unless 

 it is frequently changed.) If drops of water are put both 

 on the front lens and on the slide, there will be little trouble 

 from air bubbles, even with the concave front of the two 

 high-power, water-immersion objectives of Zeiss. 



2. Slight traces of grease soon get on the front lens, and 

 oil will float as a film on the water used for immersion. 

 Hence, after using, the front lens should be dried with a 

 small roll of blotting paper; and then cleaned with a roll 



