Solar Spectrum analogous to the Fixed Lines of Fraunhofer. 361 
through the prism is converged to a focal image on a white 
screen by the action of an achromatic lens, the spectrum which 
results is given in great purity, and Fraunhofer’s lines are 
quite apparent. The larger ones are seen by the most casual 
inspection. 
2. A tithonographic surface, after being placed in this 
spectrum, exhibits impressions of an analogous character, 
being covered with the representations of multitudes of inac- 
tive lines varying greatly in dimensions. 
3. After several attempts last summer I succeeded in dis- 
covering these lines, and have obtained impressions of them 
sufficiently perfect. 
4, Before proceeding to the description of the mode which 
is to be followed, and of the characters of the lines themselves, 
I cannot avoiding calling attention to the remarkable circum- 
stance which has frequently presented itself to me of a great 
change in the relative visibility of Fraunhofer’s lines, when 
seen at different periods. There are times at which the strong 
lines seen in a red ray are so feeble that the eye can barely catch 
them, and then again they come out as dark as though marked 
in India ink on the paper. During these changes the other 
lines may or may not undergo corresponding variations. 
The same observation equally applies to the blue and yellow 
rays. It has seemed to me that the lines in the red are more 
visible as the sun approaches the horizon, and those at the 
more refrangible end of the spectrum are obvious in the mid- 
dle of the day. 
5. A beam of the sun, passing horizontally from a heliosta 
mirror into a dark room, was received on a screen with a slit 
in its centre, the slit being formed by a pair of parallel knife 
edges, one of which was moveable by a micrometer screw ; 
the instrument being in fact the common instrument used for 
showing diffracted fringes. The screw was adjusted so as to 
give an aperture 2,inch wide,and the light passing through fell 
upon an equiangular flint-glass prism placed at the distance 
of eleven feet. Immediately on the posterior face of the prism, 
the ray was received on an achromatic lens, the object-glass 
of a telescope, and brought toa focus at a distance of six 
feet six inches, at which place an arrangement was adjusted for 
exposing white paper screens, on which the more prominent 
fixed lines might be seen and their position marked, or sensi- 
tive plates substituted for the screens, occupying precisely the 
same position. The lines on the screens could therefore be 
compared with those on the sensitive surfaces as to position 
and magnitude with considerable accuracy. In these trials 
I have generally used an achromatic lens, but the lines can be 
Phil. Mag. S. 3. Vol. 22. No. 146. May 1843. 2B 
