sensitive surface at _c_, and the
impression of the negative is there produced with a rapidity proportioned
to the light admitted, and the sensibility of the surface presented. By
varying the distances between _a_ and _c_, and _c_ and _b_, any dimension
required may be given to the positive impression. Thus, from a medium-sized
negative, I have obtained negatives four times larger than the original,
and other impressions reduced thirty times, capable of figuring on a
watch-glass, brooch, or ring.

"Undoubtedly one of the most interesting and important advantages gained by
this simple arrangement is, the power of varying the dimensions of a
picture or portrait. Collodion giving results of almost microscopic
minuteness, such negatives bear enlarging considerably without any very
perceptible deterioration in that respect. Indeed, as regards portraits,
there is a gain instead of a loss; the power of obtaining good and pleasing
likenesses appears to me decidedly increased, the facility of subsequent
enlargement permitting them to be taken sufficiently small, at a sufficient
distance (and therefore with greater rapidity and certainty) {61} to avoid
all the focal distortion so much complained of,--while the due enlargement
of a portrait taken on glass has the effect, moreover, of depriving it of
that hardness of outline so objectionable in a collodion portrait, giving
it more artistic effect, and this without quitting the perfect focal point
as has been suggested.

"But there are many other advantages obtained by this process. For copying
by engraving, &c. the exact dimension required of any picture may at once
be given to be copied from.

"A very small photographic apparatus can thus be employed when a large one
might be inconvenient or impracticable, the power of reproducing on a
larger scale being always in reserve. Independent of this power of varying
the size, positives so taken of the _same_ dimension as the negative
reproduce, as will be readily understood, much more completely the finer
and more delicate details of the negatives than positives taken by any
other process that I am acquainted with.

"The negative also may be reversed in its position at _a_ so as to produce
upon glass a positive to be seen either upon or under the glass. And while
the rapidity and facility of printing are the same as in the case of
positives taken on paper prepared with the iodide of silver, the negatives,
those on glass particularly,