190 



from a system of duplicated parts, not only do it 

 cheapest, but best. 



To Take Care of Objectives and Eyepieces. 



The utmost cleanliness must be observed with 

 objectives and eyepieces. When indistinct, dark 

 specks show in the field, the cause may usually be 

 looked for in the field lens of the eyepiece, although 

 sometimes in the eye lens also. The dust may be 

 removed by a camel's hair brush, but when this is 

 not sufficient use a well washed piece of linen, such 

 as an old handkerchief. From its fine texture 

 chamois skin is desirable, but as it is fatty it should 

 never be used until after it has been well washed. 



The same method applies to cleaning objectives. 

 Clean an immerson objective immediately after it has 

 been used, first by removing the fluid with a moist 

 linen and then by iising a dry piece, or by means 

 of Japanese lens paper. Never separate the systems 

 of objectives, even if they can be unscrewed by the 

 fingers ; it is always dangerous as they are liable 

 to become decentered. 



Keep the objectives especially in a place where 

 they are not subject to extreme and sudden changes 

 of temperature, as the unequal expansion and con- 

 traction of glass and metal may cause the cement 

 between the lenses to crack. Also keep them from 

 direct sunlight. 



