78 THREE-DIMENSIONAL PHOTOGRAPHYAt the right the camera makes two negatives (even color film
is developed as a negative before being converted into a positive)
as shown at A. From this a print is made and turned right side up
(B). However when this is done the left image is at the right and
the right image is at the left. Therefore, for the time being we
move step B to the left of the main column as shown. Restoring
the print to the upside down position of A, we cut the two pic-
tures apart and rotate each one upon its own axis, through 180'
and then we have the correct relationship as shown at C. As a
matter of fact, we do not do this. Instead we return to B at the
left of the column, cut the two pictures apart and simply lift the
right image over the left until it lies at the right as shown at C.
Thus the rotation asd the position interchange both accomplish
the end result. Rotation is what is done theoretically, the position
change or transposition is what is usually done in practice.
The two images in correct relationship C are now viewed by
the two eyes, which instead of being fixed upon a single, common
original, have an original image for each eye. Thence the process
duplicates the steps D and E as first described for direct vision.
Referring again to Figs. 5-2 and 5-3, note that the infinity image
is above center in the visual picture plane, but below center in the
negative. Images A and C in the right picture plane occupy the
lower left quadrant, while B and D are in the upper right. In
the negative these positions are just reversed. Thus we find that
the right negative is simply the right picture plane reversed to?
and bottom, right and left. The same is true of the left negative
and left picture plane.
Thus although the relative positions of all objects within the
field are inverted, the field at the left is the true left field and that
at the right is the true right field.
This brings us to a basic factor in stereoscopy. If the photograph
is identical with the image of the picture plane, the photographic
images of each object will occupy the position in the photo-
graph which the visual image occupied in the picture plane.
The two eyes do not converge upon a single image to look at
it. On the contrary, the stereogram provides a separate image for
each eye, and we look through the image, in a sense. The two
images of any given object are so spaced in the stereogram that the
visual paths from each eye to its respective image, will, if con-