Microscopical Essays. 



it otlierwife. The inftrument has this peculiar advantage, that 

 it is difficult to put any of the pieces in a place which is appro- 

 priated to another. 



Let the object be now placed either on the ftage or in the nip- 

 pers L, and in fuch manner, that it may be as nearly as pofiible 

 over the center of the ftage : bring the fpeculum F over the part 

 you mean to obferve ; then throw as much light on the fpeculum 

 as you can, by means of the mirror I L, and the double motion 

 of which it is capable ; the light received on the fpeculum is re- 

 flected by it on the object. The diftance of the lens F from the 

 object is regulated by moving the pin D up and down, until 

 a diflinct view of it is obtained. The rule which I obferve 

 is, to place the lens beyond it's focal diftance from the object:, 

 and then gradually Aide it down, till the object appears fharp and 

 well defined. The adjuftment of the lenfes to their focus, and 

 the distribution of the light on the object, are what require the 

 moft attention • on the fir ft the diftinctnefs of the virion depends ; 

 the pleafure arifing from a clear view of the parts under obferva- 

 tion, is due to the modification of the light. No precife rule can 

 be given for attaining accurately thefe points ; it is from practice 

 alone that ready habits of obtaining thefe neceffary properties can 

 be acquired ; with the affiftance of this no difficulty will be 

 found. 



Thefe microfcopes are fometimes fitted up with a toothed rack 

 and pinion, for the more ready adjuftment of the glafles to their 

 proper focus. 



P 



Descrjp- 



