ALTAZIMUTH SOLAR OBSERVATIONS. 343 
will give the relation of each wire toa truly horizontal line. In 
a second series of markings let the telescope be rotated vertically 
and mark the trace of the intersection point vertically above and 
below the original mark: this similarly gives the relation to the 
vertical line directly. By means of a protractor, or better still by 
triangular scaling and computation, the angles may be measured: 
the mean of the two, or of a series, being taken. 
18. On methods of observation generally.—There are really only 
two radically distinct methods of solar observation for the deter- 
mination of the meridian (i) the method of equal altitudes, and 
(ii) that of simple altazimuth observations. In neither can the 
centre of the sun be directly observed, so that it has to be derived 
from observations made of the position of its cireumference. The 
systems of diaphragm wires ordinarily found in theodolites are 
either AB and CD, or EG, FH and C D intersecting respectively 
at IT and J, see Fig. 4. It is always intended that AB should be 
Fig. 4. 
vertical and CD at right angles thereto; that EG and FH 
Should be symmetrically situated with respect to a vertical line 
ahs to which also CD should be perpendicular ; and that the 
intersection J should be on the wire CD, These conditions are 
but rarely even approximately fulfilled, and should be investigated: 
the defects are often quite appreciable. The sun is observed in 
Positions typified by the representations in the figure: see also 
