Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
Ch. XV] 
is a common practice in experiments with spectra to put the slit 
close to the condenser as for a lantern slide, but this lessens the 
brilliancy of the spectrum. 
Absorption Spectra 
§ 901. The apparatus being arranged as above indicated to 
project a continuous spectrum, all that remains is to insert the 
absorbing medium between the light source and the screen,- it 
makes little difference where. The appearance of the spectrum 
would be the same even if the absorbing medium were held between 
the eye of the observer and the screen. As a practical matter it is 
best to place the absorbing substance just in front of the slit. In 
this position any slight lack of planeness of its surfaces will not 
cause any interference with the optical system nor reduce the sharp¬ 
ness of the spectrum on the screen. The specimen may cover the 
entire slit, in which case the entire spectrum will show the absorp¬ 
tion bands of the substance, or the specimen may be made to cover 
part of the slit, in which case, part of the spectrum will be that of 
the light source and part will show the absorption bands of the 
specimen. The advantage of having this comparison spectrum 
of white light is to bring out much more clearly any faint absorp¬ 
tion of one end of the spectrum as with dilate copper sulphate or 
with amber glass. Liquids may best be shown by placing them 
in hollow glass boxes (fig. 365). Many variations of this method 
and many fascinating experiments will soon suggest themselves to 
the experimenter once the apparatus is set up. 
§ 902. Suitable substances.—The following substances will 
show interesting absorption bands: 
Colored glasses. Red, blue, purple, canary-yellow. 
Gelatines colored with solutions of ana- 
line dyes, for example, methyl violet, 
eosinc (red ink), fuchsine. 
Blood diluted with water. 
Solutions of mineral salts, as cobalt ni¬ 
trate in water, cobalt nitrate in alcohol or 
concentrated HC1 ; potassium permanga¬ 
Fig. 365. Glass Box for 
Absorption Spectra.


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