OBSERVING AND MANAGING ENGISCOPE. 189 



it, according to the strength of the illumination required, 

 observing always to preserve a perfect round disk of 

 light in your field of view, and every thing goes on as 

 before, the candle or lamp supplanting the day-light. 

 You may, if you please, tilt the instrument a little out of 

 the horizontal plane, for convenience of observation. 



3rd. Mode of mounting for viewing diaphanous bodies 

 by refected day-light, either in a horizontal or vertical 

 position. — For the horizontal position every thing is ar- 

 ranged in the same way as when artificial light is em- 

 ployed ; only, now the oval mirror comes into use, which 

 must be placed truly in the axis of the body, and of the 

 aperture in the stage. For this purpose, cause the mark 

 on its split socket to tally with the line drawn on the bar, 

 and then clamp the split socket tight by the screw, m : 

 then, having first removed your instrument out of the 

 direct light proceeding from the window, by placing it 

 in the middle of the apartment, proceed to reflect the 

 light given by the said window into the body of your in- 

 strument, as follows. I must premise that it will be 

 necessary that the engiscope should so present itself to- 

 wards the window that its pillar and bar should not im- 

 pede the light : thus, supposing the window is on your 

 left hand, the pillar and bar must be on your right, and 

 vice versa, so that nothing shall be bettoeen the window and 

 the mirror. Observe, that a direct lateral position of the 

 window is the most convenient and manageable. Then, 

 by means of the milled head, draw the mirror round on 



