4 
MESSES. DE LA RUE, STEWART, AND LOEWY’S RESEARCHES 
represents the projection of the plane of the ecliptic — or, what is equivalent to this, 
to ascertain the position-angle with reference to the ecliptic of the wires depicted on 
the sun-pictures, and which appear as bright lines untouched by the chemical action of 
the sun’s rays. 
If the wires in the photoheliograph could be so placed as to be always inclined at an 
angle of 45° exactly to a parallel of declination, if the declination of the sun could 
remain constant — further, if the wires could be made truly vertical to each other, and 
the picture be taken precisely in the centre of the photoheliograph, then obviously the 
wire I. (fig. 3) would make an angle of 45° with the true north point of the sun’s disk, 
wire II. of 135°, and so on — the readings on the circle giving at once the precise angles 
of position of the wires and of any other line drawn to the centre, reckoning from north 
through east. These conditions, however, cannot be fulfilled ; and the following steps 
are necessary for obtaining the corrections to be applied to the observed readings, in 
order to deduce the true position-angles. 
6. To find the true inclination (p) of the wire I. (fig. 3) to the line ND C (which is 
supposed to be drawn perpendicularly through the sun’s path, and joining its centre 
and true north point), the following method, due to Mr. Carrington, has been adopted. 
Fig. 3. 
A plate of ground glass is inserted in the chamber provided in the heliograph for 
the reception of the prepared plates for the sun-pictures ; and a bright image of the sun, 
and also of the black shadow which the wires throw upon it, in the form of well-defined 
lines, is seen on the plate. The sun is, of course, observed running along it from left to 
right, the true north limb being the upper one. In the figure the sun’s centre is sup- 
posed to run along the line EW, passing north of the intersection-point of the wires, C. 
When the centre has arrived at a point S 15 such that the length of the perpendicular 
SjAj is equal to the radius of the disk, a contact of the disk with the wire will take 
place, the instant of which is observed by a chronometer. There will be four such con- 
tacts, the times of which may be denoted by the letters A„ B„ A 2 , B 2 . 
