o minutes, and the other eye, still remaining
in the head, was then exposed at the window just as the first had been.
It was then taken out and placed in the alum solution like its fellow.
The next morning the two retinae were carefully isolated, separated from
the optic nerve, and turned. On a beautiful rose red ground a sharp
image, somewhat more than one millimetre (one-twenty-fifth inch) square
was found. The image on the first retina--that which was exposed during
life--was somewhat reddish and not so sharply defined as that on the
other.

This fixature of the last impression on the living retina is by no
means an accidental discovery, but is the final step in a laborious
series of delicate researches. Nor is it the triumph of one man alone,
the preliminary work having been performed by two distinguished
physiologists. Prof. Boll of Rome discovered that the external layer of
the retina in all living animals has a purple color, which is destroyed
by light. During life the color is perpetually restored by darkness,
but after death, Boll thought, it disappeared entirely. Prof. Kuehne
followed up this wonderful discovery and confirmed it in general, while
correcting some of Boll's conclusions. He first ascertained that death
does not necessarily destroy the color, since a retina that is not
exposed to white light, but is kept in a room lighted by a yellow
sodium flame, retains this "vision purple" for twenty-four or
twenty-eight hours, even though incipient decomposition may have set
in. It is destroyed at the temperature of boiling water or by immersion
in alcohol, glacial acetic acid, and strong solution of soda, but in
strong ammonia, saturated solution of common salt, or glycerine, it
remains undiminished for twenty-four hours. On testing the effect of
different colored lights upon this "vision purple," he found that the
most refrangible rays change it most, while red has hardly more effect
than yellow light. The color is not so delicate as Boll supposed. A few
moments' exposure to daylight does not bleach the retina. This requires
exposure for a considerable time to direct sunlight. The source of the
color was found to be the inner surface of the choroid upon which the
retina lies. If a portion of the retina is disengaged from the choroid
and raised up, it bleaches, though the remainder, still attached
portion, retains its color. If the raised flap is carefully replaced
upon the choroid, it regains its purple hue.