370 
ME. WAEEEN DE LA EEE ON THE 
one side of the table were inserted two brass plates with long slots through which two 
screws worked. These screws passed through a straight edge, which could be adjusted 
so as to cause a right-angled drawing-triangle resting against it to assume any required 
position with respect to the image of the position-wires depicted on the photograph. A 
long glass mirror was attached to the frame of the table underneath the top, in such a 
way as to be adjustable to the angle best suited to reflect light through the transparent 
positive. In front of and above the table was placed an inclined screen formed of 
tissue paper, to diminish the direct light, a certain amount of which was required to 
show the position of the etching-point. Without the aid of this screen, the direct light 
would have been too powerful, and would have prevented the details of the transparent 
photograph from being seen by the light transmitted from below after reflexion from 
the mirror. 
In the flrst place, the centre of the picture was found, and marked with a diamond 
point. A drawing- triangle, with one angle of 90° and two of 45°, was now placed over 
the photograph, with one of its sides resting against the adjustable straight edge, when 
its hypothenuse would coincide approximately in direction with the images of the wires. 
By adjusting the straight edge, the hypothenuse of the drawing-triangle was brought to 
exact coincidence with either of the wires, and the straight edge, against which it rested, 
was then (by means of the screws passing through it) clamped in position. By sliding 
the triangle along in contact with the straight edge, a line parallel with the wire was 
next set off passing through the centre, and marked slightly on the periphery of the 
picture by scratching with a diamond point through the collodion film. On taking in 
the beam-compasses a chord corresponding to 45° plus the known + or — error of the 
\vfres, a circle of right ascension, or a parallel of declination, could be made to pass 
through the centre, and, the points of its intersection with the lunar disk having been 
marked, any angles of position could be ascertained, by taking the chord between any 
part of a protuberance to be measured and the normal points thus set off. 
If it were desired to produce an etching of any photograph, the outline of the pro- 
tuberances or of the sun’s disk and spots, or of the crescent of the sun, as the case might 
be, was traced very carefully with an etching-point through the collodion, ■svith the aid 
of a lens. When this had been done, the plate was warmed by holding it before a bright 
clear fire, and a piece of composition, consisting of a mixture of parafiin and white wax, 
rubbed over it ; the heat of the plate caused the waxy mixture to melt, and thus a very 
even, thin, and translucent etching-ground was laid on the glass. The outline was now 
traced a second time, in this instance through the wax, and a camel-hair pencil, whetted 
with liquid hydrofluoric acid, was rapidly run over the parts traced. In about a minute 
the acid was removed with blotting-paper, and the plate rinsed with w’ater, and again 
dried with bibulous paper. When quite dry, the wax was melted by holding the plate 
before the fire, and wiped off with a cloth. If the etching proved satisfactory, it was 
again covered with an etching-ground, then centered on the circular dinding-engi’^e, 
and degrees and subdivisions set off, starting from a normal point previously marked on 
