322 PROJECTION APPARATUS FOR DRAWING [Cn. X 



apparatus back and forth to get the different sizes required. (For 

 getting the picture like the object see 512). 



458. Drawing on a horizontal surface. From the earliest use 

 of projection apparatus for drawing, it was the custom to draw the 

 image on a vertical surface, or by means of a plane mirror to change 

 the direction of the rays of light so that the image would fall on a 

 horizontal surface. It was found also that when a plane mirror 



FIG. 1 80. PROJECTION MICROSCOPE FROM CHEVALIER (Planche 2). 



M Mirror reflecting the sun's rays (RR 1 , rr 1 ) to the condenser (C); from 

 the condenser they pass to the substage condenser (c) and are condensed upon 

 the object (o). 



L Achromatic objective. 



A Amplifier composed of a plano-convex and a double concave lens; this 

 amplifier makes the rays much more divergent, i. e., BB' instead of bb 1 . 



P Right-angled prism acting as a 45 degree mirror to project the image 

 down upon a horizontal surface for drawing. 



was used, the image on the horizontal surface appeared erect. 

 Sometimes the mirror was placed before the objective and changed 

 the direction of the rays 90 degrees (fig. 89), and sometimes it was 

 used to bend the rays downward after passing through the objec- 

 tive. With the microscope and magic lantern the mirror is usually 

 beyond the objective (fig. 182, 193). 



Reflecting prisms have been much employed with the microscope 

 instead of mirrors (fig. 180, 192). The}- have the advantage of 

 giving more perfect reflection and of avoiding doubling of the 

 image, as occurs with a plane mirror silvered on the back. 



