DUST ON THE EYE-PIECE 437 



It may be noted that dust on the eye-piece can be detected in ;r 

 dim light, and can be discovered by closing the iris diaphragm. The 

 lens of the eye-piece on which the dust appears may be localised by 

 rotation ; and this should be done before wiping. In reference to 

 dust on the back of the objective, it should be observed that if the 

 eye-piece be removed, dust sometimes appears to be upon it which 

 comes really from the focus of the sub-stage condenser, and is, in fact, 

 not on the back of the objective at all. To find this condition, remove 

 the light modifier (if in use), for the dust may be on it, and rotate 

 the condenser ; else there will be needless and injurious rubbing of 

 the back lens of the objective. 



With oil-immersion objectives dust or air-bubbles in the oil must 

 be carefully avoided. 



If chamois leather be used for cleaning the lenses, it should be 

 previously well beaten and shaken, and then kept constantly in a 

 well-made box. 



