Till': oH.iEcriVE 127 



Care of the Objectives. — Both front, and back knis 

 surfaces of the objective reciuire regular examination with a 

 magnification of 3 or 10 times; and the front lens especially, 

 requires usually rather frequent cleaning. The front lens 

 of the 10 objective should remain clean for a long interval, 

 but the front surface of the 20 may occasionally come into 

 contact with oil or liquids. It should then be cleaned 

 with a small roll of lens paper, moistened with xylol if 

 necessary, and finally with lens paper moistened with 

 distilled water. A water-immersion objective seems to 

 get more or less greasy in time on the front surface; and, 

 in the writer's experience, this surface must be regularly 

 cleaned, first with a roll of lens paper slightly moistened 

 with xylol, and then with another roll of paper moistened 

 with distilled water. Even an oil-immersion objective 

 may well be cleaned with xylol and lens paper just before 

 use, for a slight smear of dried cedar oil may be on the front 

 surface and will certainly injure the definition. The back 

 lens of an objective catches any particles which fall down 

 the tube; especially if the eyepieces are often changed. 

 This dust must be regularly removed. The coarsest 

 particles can be removed by blowing on the back of the 

 detached objective through a long, thin glass tube; or 

 by suction from a rubber bulb. The rest of the dust the 

 writer finds (together with a film which appears in some 

 laboratories) is best removed by a roll of lens paper just 

 moistened with distilled water. Dust on a lens surface, 

 however, harms the image less than grease or a film. A 

 touch of the finger always deposits grease on clean glass, 

 and lens paper which has been touched by the fingers 

 carries this grease, to spread it as a fine film over the lens 

 surface. This is obviated by making small rolls of lens 

 paper, and using always the freshly torn end of a roll. 

 Most soiled lens surfaces should be wiped first with xylol, 

 to remove grease. 



Objectives not in use on the nosepiece are well kept, 

 in the writer's experience, out of their boxes, in an 

 ordinary dessicator over fused calcium chloride. In this 



