OBSERVING AND MANAGING ENGISCOPE. 185 



horizontal and vertical ones ; say angles of 40 and 45 

 degrees, which are comfortable and convenient for the 

 observer, though, 1 think, unfavourable to the action 

 of the illuminative part of the instrument. I shall pro- 

 ceed to give directions, on the supposition that you wish 

 to see in the simplest and best possible manner. 



1st. Manner of mounting for viewing inanimate trans- 

 parent objects by pure intercepted day -light. When the 

 instrument is first removed from its case, the circular 

 bar, c, will be locked fast into the upper or longest leg, 

 by means of its projecting pin ; it is loosened by drawing 

 the internal tube of the pillar a little out, for which pur- 

 pose, grasp the ball and socket in one hand, and the legs 

 in the other : by pulling in opposite directions, the inner 

 tube will be detached, and the pillar heightened in any 

 degree requisite for observation, at the same time the 

 bar will be left at freedom to move in any direction by 

 means of the ball and socket : immediately underneath 

 the pillar is an adjusting screw, e, by means of which the 

 pillar gains the support of a fourth leg, and is rendered 

 incapable of springing ; it can be used when great stea- 

 diness is required : it should but barely touch the table 

 or stand on which the engiscope is placed. 



Having then spread out the legs, and arranged the op- 

 tical part according to the preceding directions, attach 

 the slider-holder, fig. 25, to the stage. This is done by in- 

 serting the projecting catches into the nicks, and turning 

 it one quarter round : this fixes it fast, so that it- cannot 



