92 DR J. HALM ON 
effected by bringing a large-sized, right-angled prism into the cone of light between the 
object-glass of the heliometer and the focus. By turning the prism, the solar images 
projected upon the slit-plate could be brought into the desired position, which is 
represented in the accompanying fig. 2. I may say that in order to attain sufficient 
accuracy of this adjustment, two lines parallel to the slit and at equal distances from it 
had been engraved upon the slit-plate. The solar images were brought into such a 
position that the two lines cut off equal segments. This arrangement was quite 
sufficient to guarantee the heliographic latitudes of the points measured within a 
fraction of a degree. Such small errors in the adjustment have, however, no appreciable 
effect on the observations, because they displace the measured point on the one limb 
exactly as much towards the equator as they displace the point measured on the other 
limb towards the pole. The total displacement therefore still agrees practically with 
that we should have obtained if no such error had been present. ‘This consideration, 
however, does not apply to points exactly on the equator. 
Fic. 2. 
The heliometer employed in these observations has been kindly lent to this Observatory 
by the Hon. Lord M‘Laren, Judge of the High Court of Session of Scotland. It is the 
instrument used by Sir Davin Giz in the Mauritius Expedition, 1874, and a full 
description of it may be found in vol. 11. of the Dunecht Observatory publications. Its 
aperture is 4'2 inches, and the focal length about 64 inches. The optical quality of the 
glass is exceedingly good. For the purpose of the observations the eye-end part of the 
heliometer had to be removed, so that the focus came to fall outside the main tube. The 
instrument is mounted upon a cast-iron table, adjustable m two horizontal directions at 
right angles, with the optical axis parallel to the line of the meridian, the object-glass 
towards the north. As a collimator, a telescope is used of 4 inches aperture and 50°5 
inches focal length. The eye-end tube, which carries the slit, is adjustable by means of 
a focussing screw. ‘The cylinder of rays, emanating from the object-glass of the collima- 
tor, is thrown upon a Rowland plane grating of speculum metal, 5 inches long, 35 inches 
broad, with 14,438 lines to the inch. The surface of the grating is perpendicular to the 
axis of the collimator. I have used the spectrum of the third order, which, upon the 
one side of this grating, is remarkably bright and well-defined. The disturbing effect of 
the overlapping violet spectrum of the higher order was found to be sufficiently elim- 
