190 MICROSCOPIC ILLUSTRATIONS. 



its axis, at the bottom of the arm p, until it fronts the 

 casement ; then all that is requisite is, to turn the milled 

 head, r, and along with it the mirror, round, until its 

 light fills your field of view ; which will be at an angle 

 of about forty-five degrees, if the window is directly op- 

 posite the mirror, and your bar parallel with it. You 

 may now, if you please, use the condensing lens, n, but 

 I do not recommend it, as, along with an increase of 

 brightness, it generates a degree of confusion and nebu- 

 losity. If employed, see that it is clamped tight into its 

 true position on the bar. Like the mirror, it should be 

 as close to the stage as possible. 



If you wish to be exactly acquainted with the colour of 

 transparent bodies, use the plaster of Paris side of the 

 mirror to reflect the light of the sun, in an apartment 

 exposed to it. With this intent, place the instrument on 

 a table on which the sun falls, in such a manner, how- 

 ever, that it shall only impinge on the plaster, and leave 

 the rest of the engiscope in shade ; then proceed with it 

 as if it were day-light. This, in my opinion, is the only 

 way in which the light of the sun can be employed with 

 advantage for illumination, and it brings out colours 

 with the most perfect fidelity. 



When the instrument is used in the VERTICAL POSI- 

 TION, it is, of course, in the altitude represented in the 

 plate, and is governed precisely in the manner just de- 

 scribed. I do not like this position at all : it should, I 

 think, only be used with bodies which must be confined 

 to the stage by their gravity alone, which case may some- 



