On the Cork Equatorial Telescope, ée. 367 
in which two eye-pieces are mounted, one sliding in a groove at 
right angles to the other, so that while one has its journey back- 
wards and forwards on the horizontal line, the other has its 
journey on the vertical line, according to how the cap is placed, 
for this cap is capable of rotation to meet various circumstances. 
How to use the Instrument. 
Istly. The two stars are brought on the horizontal line, and the 
distance measured from centre to centre along that line. This dis- 
tance is measured by counting the number of spaces on the glass, 
adding the residue as measured by the micrometer screw. Thus 
the screw is never used for larger measures than 5 inch, and 
therefore errors of screw and temperature errors are much reduced. 
In bisecting; one star is brought into the field of the eye-piece, and 
a bisection is made with one of the diamond lines by moving the 
micrometer by one or other of its slipping piece screws. Then 
the other eye-piece is moved till the second star is seen, and a 
bisection is made with the nearest spider line by moving the micro- 
meter head. Then the eye can be moved back to eye-piece, and 
the bisection checked, and again back to first eye-piece. When 
it is seen that both are satisfactory the measure can be read off. 
2ndly. The micrometer is turned round till the horizontal line 
becomes parallel to the path of apparent motion of the star. This 
is easily found by stopping the clock and allowing the star to 
run along the horizontal wire. Now the other star will be found 
to cross the vertical line somewhere, while the first star is on the 
horizontal line. This second star is then bisected on the vertical 
line, while the first star is bisected by one of the spider lines; thus 
the difference in M is found. We then have two sides of a right 
angle triangle, and, of course, all the elements are known.* 
To ascertain the errors (if any) of the distance of the lines of 
course, the usual plan of taking transits can be adopted, and to 
ascertain if the line be perfectly at right angles a special additional 
eye-piece is provided, so that transits can be taken across each 
diagonal of the square. 
So far I have spoken of instruments, which are either com- 
pleted or nearly so. I now proceed to say afew words concerning 
matters that are as yet only projected or in progress. On these 
* The words horizontal and vertical are here only used in relative terms, and for the 
sake of explanation. 
