188 MICROSCOPIC ILLUSTRATIONS. 



the instrument just described, is by far the best and 

 simplest with which I am acquainted for all sorts of 

 transparent objects, and should, therefore, always be 

 used when practicable ; and this observation applies to 

 all sorts of microscopes and engiscopes. Occasionally, 

 however, the weather is so dull and gloomy, that we see 

 better by artificial light : I therefore now give the 



2nd. Method of mounting for viewing transparent objects 

 by artificial light. Setting out on the supposition that 

 the instrument has been just removed from its case, and 

 is in the position given in the plate, proceed as follows : 

 Place it on a table ; detach the bar from the front leg, 

 as before ; move the mirror and condensing lens to one 

 side ; then turn the bar round till it assumes a hori- 

 zontal position, keeping the neck of the ball at the bottom 

 of the quadrant slit, in the position in which it is repre- 

 sented in the drawing : its rotatory action is the only one 

 at present required. This arrangement will leave the 

 space, both behind and before the stage, open for the in- 

 troduction of a lamp or candle, &c. The condensing 

 compound lens, 'f, attached to the body, together with the 

 plane-convex one, n, under the stage, and the mirror, 

 must remain inert and unemployed, in this as in the pre- 

 ceding mode of observation, and may be removed alto- 

 gether. Adjust the height of the instrument to suit that 

 of your light and eye, by drawing the tube of the pillar 

 in or out : place your light centrically behind the aper- 

 ture of the stage, at two, four, or six inches distant from 



