great uniformity and be
quite permanent.

Bromide of Lime, another accelerator, can be used in the same manner,
except it is, only necessary, when a solid sensitive is used, to mix it
with the sand without placing it in a bottle. This method is employed
with great success by a few, who have regarded it as a secret worth
keeping.

A Combination, requiring the Use of only One Coating-box.--It is often
wondered by beginners, why some solution requiring only one coating
cannot be employed. This can be done, but the results are not so
satisfactory as when two or more are employed. Such an accelerator may
be produced by adding alcoholic solution of iodine to a solution of
chlorate of potash, until the latter will take up no more of the
former, and to each ounce, by measure of this solution, ten drops of a
saturated solution of bromide in water are added. The solution of
chlorate of potash is made by diluting, one part of a saturated
solution of the salt with ten parts of water. The use of the chlorate
is simply as a solvent of iodine.

Fats as Accelerators.--The use of fats, oils, or greasy substances, 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.

Light--Optics--Solar Spectrum--Decomposition of Light--Light, Heat, and
Actinism--Blue Paper and Color for the Walls of the Operating
Ro