In computing the weight of the various items for a photographic tour, the
glass almost invariably comes out at the head of the list, and the farther
or longer the journey, so much more does the weight of the plates stand out
pre-eminent; indeed, if one goes out on a trip with only three dozen
half-plates, the glass will probably weigh nearly as much as camera, backs,
and tripod, in spite of the stipulation with the maker to supply plates on
"thin glass."

Next in importance to glass as a support comes paper, and it is quite easy
to understand that the tourist in out of the way parts might be able to
take an apparatus containing a roll of sensitive paper, when it would be
altogether impracticable for him to take an equivalent surface of coated
glass, and in such a case the roller slide becomes of especial value.

The roller slide of Melhuish is tolerably well known, and is, we believe,
now obtainable as an article of commerce. The slide is fitted up with two
rollers, _a a_, and the sensitive sheets, _b b_, are gummed together,
making one long band, the ends of which are gummed to pieces of paper
always kept on the rollers. The sensitive sheets are wound off the left or
reserve roller on to the right or exposed roller, until all are exposed.

[Illustration]

The rollers are supported on springs, _a a_, to render their motion
equal; they are turned by the milled heads, _m m_, and clamped when each
fresh sheet is brought into position by the nuts, _a squared a squared_. _c_, is a board
which is pressed forward by springs, _c c_, so as to hold the sheet to be
exposed, and keep it smooth against the plate of glass, _d_; when the sheet
has been exposed, the board is drawn back from the glass in order to
release the exposed sheet, and allow it to be rolled on the exposed roller;
the board is kept back while this is being done by turning the square rod,
_c squared_, half round, so that the angles of the square will not pass back
through the square opening until again turned opposite to it; _e e_ are
doors, by opening which the operator can see (through the yellow glass, _y
y_) to adjust the position of the sensitive sheets when changing them.

The remarkable similarity of such a slide to the automatic printing-frame
described last week will strike the reader; and, like the printing-frame,
it possesses the advantage of speed in working--no small consideration to
the photographer in a distant, and possibly hostile, country.