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On the Cork Equatorial Telescope, €e. 365 
The Micrometeis.—It is intended to have two micrometers for 
use with the equatorial. 
The Bi-filar Micrometer is of the ordinary construction. Two 
screws each of fifty threads to the inch, carry one or more spider 
lines each. These wires can be made to traverse the whole 
breadth of the field, so that measures can be taken in both 
directions. Whole turns of the screw are read off through the 
windows in front, and zo parts of a turn on the micrometer 
heads. 
The Duplex Micrometer (Fig. 12),is an instrument of novel 
construction, only one other being in existence. This one other is 
an instrument which I made about ten months since for the 
Savillian Observatory in Oxford, and it has been in constant use 
there, and has given the most satisfactory results, 
The object or special use for the Duplex Micrometer is that of 
measuring the distances and positions of stars which are too 
far asunder to allow them to be brought into the field of an 
ordinary micrometer. 
For this purpose up to the present the instrument called the 
Heliometer has been used, but this involves the use of a special 
