Microscopical Essays, ^ 



and the flame of the lamp, depends on a combination of circum- 

 itances, which a little practice will difcover. 



2. To regulate the guide for the eye, or to place the center of 

 the eye-piece L, fo that it may coincide with the focal point of 

 the lenfes, and the axis of vifion. Lengthen and fhorten the 

 tubes M N by drawing out or pufhing in the inner tube, and 

 raifmg or deprefling the eye-piece ML, till you find the large 

 lens (which is placed at the end AB of the wooden body) filled by 

 an uniform field of light, without any prifmatic colours round the 

 edge ; for till this piece is properly fixed, the circle of light will 

 be very final 1, and only occupy a part of the lens : the eye muft 

 be kept at the center of the eye-piece L, during the whole of the 

 operation; which may be rendered fomewhat eafier to the obferver, 

 on the firft ufe of the inflrument, if he holds a piece of white 

 paper parallel to the large lens, removing it from, or bringing it 

 nearer to them, till he finds the place, where a lucid circle, which 

 he will perceive on the paper, is brighter! and moft diftinct., then 

 to fix the center of the eye-piece to coincide with that fpot ; after 

 -which a very final 1 adjustment will fet it perfectly right 



3. To adjuft the lenfes to their focal diflance. This is effected 

 by turning the pinion a, the eye being at the fame time at the eye- 

 piece L. I often place the grey glafs before the large lenfes, 

 while I am regulating the guide for the eye, and adjusting for 

 the focal diftance. 



If the obferver, in the procefs of his examination of an object, 

 advances rapidly from a (hallow to a deep magnifier, he will fave 

 himfelf fome labour by pulling out the internal tube at O, 



K 2 The 



