120 A SOUTHERN OBSERVATORY. 
like Bessel, exchanged lucrative mercantile pursuits for the compara- 
tively scanty emoluments awaiting the votaries of the stars. The 
“patines of bright gold,” with which Urania’s treasure chests overflow, 
are not of terrestial coinage. 
The distance of the sun was the first problem upon which Dr, Gill 
delivered a substantial attack; and his solution of it still remains the 
best obtained by celestial trigonometry, corresponding so closely with 
Newcomb’s value of the same great unit, derived from direct measure- 
ment of the velocity of light, as to reduce within reassuringly narrow 
limits the uncomfortable margin of uncertainty left by the transits of 
Venus. In the observations of Mars made for this purpose at Ascen- 
sion in 1877,* Dr. Gill employed the instrument of his predilection, 
called—on the lucus & non lucendo principle—a ‘“ heliometer.” 
A heliometer is a telescope of which the object glass has been sawn 
in two. This does not sound like, nor would it be, an improvement for 
purposes of simple star-gazing; but the end in view is different. It is 
that of precisely determining the angular distances between adjacent 
stars, or between a planeteand stars near it, though in many cases be- 
yond the range of the ordinary micrometer. The following is the way 
in which this end is compassed. 
The half lenses of the object glass are separable by a very fine screw 
motion, and they form independent and separable images of any object 
upon which the telescope is pointed. These images unite into one when 
the two segments unite to complete one circle; as they are made to 
slide apart, the images to slip sideways asunder, to an extent which can 
be measured with the minutest accuracy by exquisitely divided scales 
read with a powerful microscope. In the actual process of observation, 
the telescope is fixed upon a point midway between the stars under 
scrutiny, so that the field of view is, to begin with, empty. Neither star 
can-be seen. Then the segments of the object glass are moved oppo- 
sitely along a line brought beforehand to agree with the line of direc- 
tion between the stars, until the more westerly (say) of the pair as 
imaged by one segment, and the more easterly as imaged by the other, 
begin simultaneously to appear, and are at last carefully made to coin- 
cide in the middle of the field. After the scales have been read, the 
motion is reversed, and a similar coincidence is brought about between 
the oppositely corresponding stars—that is, between the easterly mem- 
ber of the pair shown by segment No. 1, and the westerly member of 
the pair shown by segment No. 2. The total distance traversed is, of 
course, equal to twice the distance between the stars. 
The refinements, however (which can not here be explained), attend- 
ant upon these operations are what make their results valuable, and 
the process of educing them laborious. With the Copernican “tri- 
quetrum ” the measured apparent intervals between any two of the 
* For a popular account of the e xpe dition, see Mrs. Gill’s ¢ charming “ Six Months in 
Ascension.” Murray. Second edition. 1880. 
