Dr. J. Hopkinson in his dispersion photometer uses a double convex lens.
This gives a focal image of the arc-lamp between the lens and screen,
whence the rays diverge very rapidly, thus giving the desired dispersion
effect.
It is principally for arc lamps that dispersion photometers are used.
Photometer, Shadow.
A photometer in which the relative intensity of the two lights is
estimated by the intensity or strength of shadows of the same object
which they respectively cast.
Fig. 262. RUMFORD'S SHADOW PHOTOMETER.
Fig. 263. RUMFORD'S SHADOW PHOTOMETER ARRANGED FOR TESTING
INCANDESCENT LAMPS.