236 REPORTS FROM THE SECTIONS. ‘ 
The Triangle Micrometer. 
By H. C. Russet, B.A., F.R.A.S. 
[Read before the Astronomical Section, 6 September, 1878.] 
Most observers who have had to observe the positions of small bodies, 
comets for instance, have learned to dislike the ring micrometer ; 
for although it gives differences of time with tolerable accuracy, the 
differences in declination are pe unsatisfactory when found an 
very troublesome to compute ; when therefore I had occasion to 
use it recently the old feeling of f dissatisfaction came back to me, 
and set me thinking whether some more convenient form might 
not be devised, and the result I propose to lay before you to- 
night. 
Youare aware that Lacaille’s observations at the Cape were 
made with rhomboidal figures, that is, two triangles base to base 
in the focus of the object-glass of the telescope. This form is much 
more satisfactory and convenient than the ring, yet I cannot find 
that any one has suggested its use in preference to the ring, never- 
theless the advantages are many and they are still further incre 
by making the figure a triangle. I therefore adopted the one in 
which the base of the triangle is exactly equal to the height, which 
is the most convenient form for the reduction of the results, and 
has, as you will at once see, this property,— that the difference in 
time of transit between two bodies crossing it, converted into 
is their difference in declination. Of course, if the observations 
aidion, the declinations are as valuable as the right ascensions, 
which as we have seen is not the case in the ring micrometer. In 
use the triangle is very convenient : by es a pasbeging’ along 
the base while the instrument is at rest it can ce be set 
position, so that its base is a oh ong of Paaenthertieny ae its per- 
pendicular one of right asce 
If the transits are Seated: at ingress and ond for each side, 
four transits are obtained from which to get the mean. 
The sides of the triangle may be tested by io owing a star to 
run along, which if they are not straight will at once show the 
irregularity. . 
