OPTICS. 139 
right angles to each other, then the colors will become enfeebled by 
rotating the upper mirror. When the angle of rotation amounts to 45°, 
then the colors will disappear almost altogether. Continuing the rotation 
beyond 45°, the colors will again appear, becoming brightest at 90°, and 
fading away again to 185°. The second series of colors will, however, be: 
complementary to the first; thus, for red we shall have green, for yellow, 
blue, and inversely. 
The colored rings of polarization are best seen by means of the apparatus 
represented in fig. 73. This was invented by Soleil, and is well calculated 
for accurate measurements. Here there are three convex lenses, 0, c, d, 
each one of about one inch focus; the two first, b and c, are separated 
by the sum of their foci, and in their common focus there is a plate of 
crystal, 7, in a frame, turning on a horizontal axis: a@ is the polarizing 
mirror. Parallel rays falling upon and polarized by this mirror, pass 
through the lens 6, converge towards the crystal, and strike the lens ¢: 
from this they emerge again, parallel, and striking the third lens, d, are 
again rendered convergent. A micrometer is placed between the lenses 
cand d; ¢ is a tourmaline plate serving as a disperser. 
1. Of the Chemical Action of Light. 
The blackening of chloride of silver, caused by the action of light, was 
early suggested as one means of fixing the beautiful images of the Camera 
Lucida. Experiments on this subject failed, however, until recently, when 
the object was accomplished by Messrs. Niepce and Daguerre in France, 
and Fox Talbot in England. The former operators made use of a plate of 
copper, plated with silver, as the best surface to receive the impression of 
light. An exceedingly high polish is given to the silvered surface, which is 
then to be carefully cleaned and exposed to the vapor of iodine, until a 
deep golden yellow layer of iodide of silver has been formed. The plate is 
now ready to receive the impression. This is produced by means of a 
camera obscura of a construction especially adapted to the purpose. It 
consists of a square box, with a horizontal tube in the centre of one end, 
in which is placed a good achromatic lens of about ten inches focus ; at the 
opposite extremity of the box there is a groove in which slides a plate of 
ground glass. Ihe object whose image is to be transferred to the plate, is 
then to be brought before the camera, and its image, well defined and of the 
proper size, made to fall on the plate of ground glass. This is then to be 
vemoved and the prepared plate substituted, and allowed to remain for 
several seconds. On removing the plate no apparent effect will have been 
produced ; the picture, however, has been made, although invisible. To 
bring it out, the plate is suspended in a dark box over a vessel of mercury, 
gently heated from beneath by a spirit lamp. The vapor of the mercury 
will slowly rise and adhere in the form of very minute globules to the parts 
of the picture acted on by light, the remaining portions not being affected. 
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