184 MICROSCOPIC ILLUSTRATIONS. 



eye -piece (this is a subject which I shall recur tc* 

 hereafter.) 



When you clean the eye-glasses (a point of great im- 

 portance to pure vision) do not remove more than one 

 at a time, and be sure to replace it before you begin 

 with another : by these means you will be sure to pre- 

 serve the component glasses in their proper places : 

 recollect that if they become intermingled, they will be 

 useless. Keep a piece of well-dusted chamois leather, 

 slightly impregnated with some of the finest putty or 

 crocus powder, in a little box to wipe them with (for it is 

 of consequence to preserve it from dust and damp), 

 the former will scratch the glasses, and the latter pre- 

 vent you from wiping them clean. As to the object- 

 glasses, endeavour to keep them as clean as possible 

 without wiping, and merely use a camel's hair pencil to 

 brush them with ; for wiping them hard with any thing 

 has always a tendency to destroy their adjustment, unless 

 they are firmly burnished into their cells. 



Now I shall suppose that you want to view some trans- 

 parent inanimate object mounted in a slider. There are 

 several ways of doing this, according to the position in 

 which you choose to preserve your instrument, whether 

 directed upwards towards the sky, horizontally, or ver- 

 tically, looking downwards, &c. The first is the best 

 method for direct day-light; the second, for lamp or 

 candle-light ; the last is the worst of all for either, and 

 should not be resorted to when the others can be used. 

 There are, however, intermediate positions between the 



