148 



for convenience in use. A small bottle is gener- 

 ally supplied with each oil immersion objective. 

 Grent care sJiould be used in keeping it free from 

 dust, as it often happens that an objective fails to 

 give satisfaction, due to a small particle of dust 

 which may float in the fluid before the front lens. 

 Great care should also be exercised in applying oil 

 to the front lens and after the application, it is 

 strongly recommended to examine it with a mag- 

 nifier , that there may be no air bubbles present. 

 These as well as dust may seriously interfere with 

 obtaining satisfactory results. If bubbles are 

 present the oil should be removed and a fresh 

 quantity applied. Care should also be taken not to 

 apply too great a quantity. After the stopper has 

 been withdrawn from the oil, allow the oil to run 

 down the rod or brush until the last natural drop 

 has separated from it and apply the remainder, or 

 less than a drop, to the front of the objective. 



Attach the objective and lower it until the fluid 

 comes in contact with the cover and observe this 

 by lowering the head. Focus as with dry objec- 

 tives. The use of immersion fluid in itself involves 

 a certain amount of inconvenience, but the obser- 

 vance of fixed rules will materially help to over- 

 come some of the disagreeable features. Extreme 

 cleanliness should be observed with it. After the 

 work has been completed the objective should be 

 removed from the stand and its front as well as 



