Page 469 - 89 history of astronomy_opt

Just in front of this plate there is another
slit parallel to the first, in the position where
the image of the first slit formed by the K
line of calcium falls. Thus is obtained a
photograph of the section of the sun, made
by the first slit, only in K light. As the im-
age of the sun passes over the first slit the
photographic plate is moved at the same
rate and in the same direction behind the
469