, he says, of making the pictures appear to stand out unnaturally;
that is to say, if the cameras were to be placed 12 inches apart (which
would be equal to 1 in 8), the pictures would seem to be in greater relief
than the objects.

I find that the pictures on a French stereoscopic slide I have by me have
been taken at an angle of 10 deg., or 1 in 6. This was evidently photographed
at a considerable distance, the triumphal arch in the Place de Carousel (of
which it is a representation) being reduced to about 11/4 inch in height. How
comes it then that the angle is here increased to 10 deg. from 61/2 deg., or to 1 in
6 from 1 in 9.

Moreover, the only work I have been able to obtain on the mode of taking
stereoscopic pictures, lays it down that all portraits, or near objects,
should be taken under an angle of 15 deg., or, as it says, 1 in 5; that is, if
the camera is 20 feet from the sitter, the distance between its first and
second position (supposing only one to be used) should not exceed 4 feet:
otherwise, adds the author, "the stereosity will appear unnaturally great."

When two cameras are employed, the instructions proceed to state that the
distance between them would be about 1/10th of the distance from the part
of the object focussed. The example given is a group of portraits, and the
angle, 1 in 10, is afterwards spoken of as being equivalent to an arc of
10 deg.

Farther on, we are told that "the angle should be lessened as the distance
between the nearest and farthest objects increase. Example: if the farthest
object be twice as far from the camera as the near object, the angle should
be 5 deg. to a central point between these two.

Now, I find by calculation that the measurements and the angle here
mentioned by no means {506} agree. For instance, an angle of 15 deg. is spoken
of as being equivalent to the measurement 1 in 5. An angle of 10 deg. is said,
or implied, to be the same as 1 in 10. This is far from being the fact.
According to my calculations, the following are the real equivalents:--

An angle of 15 deg. is equal to 1 in 4.
" 12 deg. " 1 in 5.
" 10 deg. " 1 in 6.
" 6-1/2 deg. " 1 in 9.
" 6 deg. " 1 in 10.
" 5 deg. " 1 in 12.
" 4 deg. " 1 in 15.

Will any of your readers oblige me by solving the above anomalies, and by
giving the proper angles or measurement u