d stops
on certain classes of subjects, and it remains with you only to guess
correctly to what class the view you wish to take belongs; I can assure
you from my own experience that there is enough uncertainty about that
point to prevent good negatives ever becoming monotonous.

The only aid I can suggest in this case is the continual use of a
note-book. Note every plate you expose, and when you have a failure be
careful to record the fact, and you will gradually find these accumulated
notes becoming a great help in cases of doubt. One hint I can give to
beginners is that a great number of the pictures to be met with in this
part of the country are intermediate between "Open Landscape" and
"Landscape with heavy foliage in foreground;" and it is scarcely needful
to say that if you are in doubt, let the exposure be rather too much than
too little; you _may_ make a negative of an overexposed plate, but never
of an underexposed one.

* * * * *

ISOCHROMATIC PHOTOGRAPHY.

[Footnote: Read at the stated meeting of the Franklin Institute, March
18, 1885.]

By FRED. E. IVES.

It is well known that the ordinary photographic processes do not
reproduce colors in the true proportion of their brightness. Violet and
blue photograph too light; green, yellow, orange and red, too dark. For a
long time it was believed to be impossible to remedy this defect; and
even when it became known that bromide of silver could be made more
sensitive to yellow and red by staining it with certain dyes, the subject
received very little attention, because it was also known that the
increase of sensitiveness was too slight to be of practical value in
commercial photography.

Dr. H.W. Vogel, who was one of the first, though not the first, to devote
attention to this subject, announced, in 1873, that he had succeeded in
making a yellow object photograph lighter than a blue or violet one, by
using a silver-bromide plate stained with coraline, and exposed through a
yellow glass. The plate showed no increased sensitiveness to red, and the
experiment, although of considerable scientific interest, did not
indicate a practically useful process.

In the spring of 1878 I became interested in this subject, and tried to
discover a method of producing plates which should be sensitive to all
colors, and capable of reproducing them in the true proportion of their
brightness. I commenced by trying nearly all the color sens