ON THE KEW OBSERVATORY. 185 
its sliding plate AS (vide fig. 1. Plate II.), with a little handle for withdraw- 
ing it in order to expose the photo-plate to the action of the iodine, &e. 
The Burning-off and Fixing Stand. 
A (fig. 6.) is a heavy mahogany board. 
αἱ, a milled-headed screw passing through A and projecting (about half 
an inch) below it. . 
a? a2, two little brass feet projecting (about the same quantity) below. 
B is another heavy mahogany board. 
6', another screw similar to αἱ and pressing on A. 
62, one of two feet similar to a? a®, also resting on A. 
C C, two tubular pillars fixed upon B. 
οἱ 6', two wires attached perpendicularly to caps on C C, which caps can 
turn on their axes. 
Y is a photo-plate resting on οἱ c!. 
The axes of a? a? are in a plane perpendicular to the plane of the axes of 
i262. This stand allows of much more rapid adjustment for horizontality 
than the usual stand having three adjusting screws. 
About the end of the month of February last, I think, Sir John Herschel 
proposed a very ingenious method of procuring surfaces in relief (as in wood 
engravings) on gelatine paper, which should exactly coincide with impressions 
procured on the gelatine by photographic means, in order that they might be 
employed in printing. 
On hearing of this, I suggested to Colonel Sabine the expediency of en- 
graving gelatine paper as if it were actually copper, with the figures of the 
magnetic and other curyes on our Daguerreotype plates, by using the gela- 
tine as tracing-paper commonly is used for the purpose of copying drawings, 
&c., and also of employing such engraved gelatine as copper plates are em- 
ployed for printing any required number of copies of such magnetic and 
other curves. 
The experiment succeeded on the first trial. 
Specimens are preserved in our Journal of gelatine paper thus engraved, 
and of declination, horizontal force, and barometric curves as printed from 
the gelatine. 
The ordinate board has been slightly modified to render it useful in this 
process. 
The method of Sir John Herschel, however, will certainly be found far 
preferable to this, when chemical difficulties have been conquered, as I sin- 
cerely hope they will be. 
In order to correct certain errors of the clock’s rate—errors arising from 
expansion, &c.,—it has been found necessary sometimes to divide the time- 
scale belonging to the curves produced into equal parts. An instrument, 
correct in principle at least, and which I hope to render an accurate and 
generally applicable mathematical instrument, has been experimented upon. 
Its principle of action is that of a well-known instrument called the “ Lazy- 
back,” and will be instantly understood by reference to Plate III. figs. 2 and 3 
(the perpendicular rods are fixed to the lower joints, but slide through the 
upper ones). 
It is evident that the points of this instrument cannot be brought-very 
close to each other: if, therefore, minute divisions of a scale are required, 
the first large division may be subdivided by means of a pair of parallel 
dividers (figs. 4 and 5), which, it may be readily seen, is an instrument 
constructed on similar principles to the above, but allowing the points to 
touch each other, The first large division having been so subdivided, the 
