109 



In using a screw-driver, grind its two large sur- 

 faces so that they are parallel and not wedge-shape, 

 and so it will exactly fit in the slot of the screw-head. 



In inclining the stand always grasp it at the arm 

 and never at the tube, as in the latter case it may 

 loosen the slide or tear off some of the parts. 



When repairs or alterations are necessary, always 

 have these made by the manufacturers ; they can, 

 from the system of duplicated parts, not only do it 

 cheapest, but best. 



To Take Care of Objectives and Eye- 

 Pieces. It is as necessary to keep these free from 

 dust as the stand, in fact even greater cleanliness 

 should be observed. When indistinct, dark specks 

 show in the field, the cause may usually be looked for 

 in the field-lens, 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 immersion objective invariably after it has 

 been used, first by removing the fluid by a moist linen 

 and then by using a dry piece. 



Keep the objectives especially in such a place 

 where they are not subject to extreme and sudden 

 changes of temperature, as the unequal expansion 



