Phenomena of Polarized Light. 91 
highly stretched becomes almost transparent, and when viewed 
by polarized light it gives when in this constrained condition a 
efinite system of colors, not unlike the figures produced ina 
circular piece of tempered glass. 
On a mode of preparing figures for exhibition upon screens by 
the apparatds of Soleil_—T here are no exhibitions within the 
whole range of optics so fascinating as those with the apparatus of 
Soleil, in which the various figures, systems of rings and colors by 
polarized light from crystals, tempered glass and other substances, 
are thrown upon a large screen in a darkened room. Among the 
many objects accompanying polarizing apparatus received in this 
country from Europe, there are usually one or two tasteful dia- 
grams representing a flower, gothic window of stained glass, or 
some pleasing device. ‘These pieces are prepared at vast labor 
and expense, and are in reality mosaic work, of thin pieces of 
selenite cemented to glass by Canada balsam. The selenite is 
k the character of the diagram or form to be represented, are lai 
upon glass as before stated. But as each piece will give two dis- 
; tinct complementary colors in one revolution of the eye-piece, 
£15 to £20. Dr. Brewster suggested a mode of making such fig- 
ures upon selenite, by scratching out the surface, and afterwards 
polishing it; but this would be more difficult of execution than 
the mosaic work. It occurred to me that a good effect might be 
produced by engraving in a peculiar way upon mica, and my first 
trials about three years since were quite successful and encourag- 
ing. The modus operandi is as follows: A piece of very clear 
and uniform mica is selected, about ,’zth of an inch thick, and 
this is examined by polarized light to ascertain its color. If the 
mica splits well we may have the ground-work of almost any 
color desired, by gradually removing very thin layers and repeat- 
ing the examination each time. e mica is then laid and sé- 
cured in place over a drawing or engraving of a bird, flower, but- 
terfly, or any object fancy selects. The whole outline of the 
gure is then traced upon the mica with a sharp steel point, ta- 
king pains to use uniform pressure. The entire film of mica 
eitcumseribed by the tracer is then removed by lifting one edge 
