POSITION OF OBSERVER. 285 



difficult class of objects, such as are recognized as the 

 severest tests. It will be much better for the student 

 if he confine himself to the instructions already given , 

 until he shall have become perfectly at home with his. 

 objectives and mounts before paying attention to what 

 we shall here have to offer. In the remainder of the 

 lessons the education of the eye will be an indispensable 

 condition to success, and the instructions can only be 

 understood acceptably by those who have paid rigid 

 attention to the preceding- instructions. 



Arrange the stand with the inch ocular, lamp and 

 mirror, just as described in lesson first. Get out the 

 large bull's eye condenser and interpose the same be- 

 tween the lamp and mirror, the flat side towards the 

 lamp, and within a couple of inches from its chimney , 

 adjust the height of the condenser so as to throw the 

 light on to the mirror. The beam from the lairm may 

 be slightly inclined upwards, but never downwards. A 

 horizontal beam is not objectionable. Now experiment 

 a little. Try and get the lamp as closely to the left of 

 the stage as practicable, leaving room to work the con- 

 denser. Next, place the slide of Cherryfield or Mon- 

 mouth on the stage. Now reflect for a moment. Where 

 is the point of maximum aperture of your objective? 

 And how will the cover of your mount respond to it? 

 If its point of maximum aperture be at nearly " closed," 

 and your cover ot such a thickness as will allow the 

 glass to "correct" nearly at closed with glycerine, then 

 you are to use the same. If so be, however, that the 

 cover is thick enough thus to cause the glass to correct 



