sed, an oblique position of a plate 
on the slide is most favorable to reflect the light into the instru- 
time exhibiting color; while 
ae thinnest glass in the same condition showed both in a high 
ployed would show, the angles of perfect 
hexagons, 120° (see figure). The method — 
‘used was to draw them upon paper under the camera lucida, — 
. 
