179 
Natural Philosophy^ — Optics. 
the spectrum when formed by all bodies, whether solid or fluid, 
is crossed with above 500 dark lines of various magnitudesj 
parallel to one another, and perpendicular to the length of the 
spectrum. He found also, that these lines had fixed positions, 
and by measuring the distance between the most prominent in 
different spectra, he obtained precise measures of the action of 
the prism on the rays that formed the corresponding coloured 
spaces. The spectra produced by the two polarised pencils of 
Iceland spar, exhibited the same lines. In order to see these 
lines, prisms perfectly free of veins must be used, and all ex- 
traneous light carefully excluded. The experiments of M. 
Frauenhofer were first published in the Memoirs qf the Aca- 
demy of Bavaria^ for 1815. 
15. Frauenhofer^ s Experiments on the illuminating Power 
of the Prismatic Rays. — -By means of an ingenious photometer, 
M. Frauenhofer measured, with great care, the illuminating 
power of the different coloured spaces, and obtained results 
Very different from those usually given i On each side of the 
yellow spaceij the light varied with very great rapidity, as ap- 
pears from the following measures : ^ 
Intensity of Light. 
At the degree * of the Med, - - 0.032, 
At the 34th degree of the Red, - - 0.094 
At the 2 2d degree of the Orange, - - 0.64 
At the lOth degree of the Yellow, - = 1.000 
At the 42d degree of the Yellow, - - 0.48 
At the 2d degree of the Blue, - - 0.17 
At the 16th degree of the Indigo, - 0.031 
At the 43d degree of the Violet, - - 0.0056 
The measures here given have no relation to the colours oppo^ 
site to themj as the colours are mentioned merely to point out 
the position in Newton’s spectrum, corresponding to the position 
in Frauenhofer’s spectrum, where the intensity of illumination 
was measured. The colours in Fraitenhofer’s spectrum, indeed, 
do not correspond with those of Newton. 
16. Frauenhofer'' s Experiments on the Inflexion of Light — • 
M. Frauenhofer has recently printed a very interesting dissei% 
* The whole length of the spectrum is here supposed to be divided into 360° ; the 
tied occupying 45° ; the Orange 27° ; the Yellow 48° | the Green 60° ; the Blue 
60° ; the Indigo 40° ; and the Violet 80°. 
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