e
more firmly of the correctness of the propositions I have advocated, viz.:

1st. That circumstances _may_ and _do_ arise in which a better result is
obtained in producing stereographs, when the chord of the angle of
generation is more or less than 2-1/2 inches.

2nd. That the positions of the camera should _not_ be parallel but radial.

I certainly thought that I had, as I intended, expressed the fact that I
treat the cameras _precisely as two eyes_, and moreover I still contend
that they should be so treated; my object being to present to each eye
_exactly such a picture and in such a direction as would be presented under
certain circumstances_. The plane of delineation being a flat, instead of a
curved surface, has nothing whatever to do with this point, because the
curves of the retinas are not portions of one curve having a common centre,
but each having its own centre in the axis of the pupil. That a plane
surface for receiving the image is not so good as a spherical one would be,
is not disputed; but this observation applies to photographs _universally_,
and is only put up with as the lesser of two evils. A plane surface
necessarily contracts the field of view to such a space as could be cut out
of the periphery of a hollow sphere, the versed sine of which bears but a
small ratio to its chord.

There is another misunderstanding into which my opponent has fallen, viz.
the part of the object to be delineated, which should form the centre of
radiation, is not the most contiguous visible point, but the most remote
principal point of observation. I perceive that this is the case from two
illustrations he was kind enough to forward me, being stereographs of a
[T-square] square, placed with the points of junction towards the observer,
and the tail receding from him; and in one case the angle of the square is
made the centre of radiation, and while its distance from the camera is
only six feet, the points of delineation are no less than three feet apart.

To push an argument to the extreme to test its value, is quite right; but
this goes far beyond the extreme, if I may be allowed such a very Hibernian
expression.

No object, however minute, can be clearly seen if brought nearer to the
eyes than a certain point, because it will be what is technically called
out of focus. It is true that this point differs in different individuals,
but the _average distance_ of healthy vision is 10 inches. Now, adopting
MR. MERRITT'