has been one of the most emphatic prohibitions about the
Daguerreotype plate. Yet it has been proved that its presence
in a small quantity upon the silver surface has the effect
of reducing the time of exposure in the camera from two-thirds
to three-fourths. An application may be made as follows:
Pour sweet oil, or rub beef or mutton fat, on a common buff,
which is free from all polishing powders. With this, buff a
well-cleaned plate, and it will leave a scum, which should be
mostly removed by using another buff, which should be clean.
Coat the plate in the usual manner, and the result will be
a great reduction in the time of exposure in the camera.
The impression produced upon a plate so prepared presents,
when coming from the vapor of mercury, a grey, scummy appearance,
which, on the application of heat in gilding, does not improve;
hence its use is not generally adopted.

We have instituted some investigations upon this subject,
and in the present volume, we shall not refer to it further.
Those wishing to learn more fully the effect of light upon
organic substances will find Robert Hunt's "Researches on Light"
an invaluable work.




LIGHT AND OPTICS.

CHAPTER IV.