366 Scientific Proceedings, Royal Dublin Society. 
telescope with a divided object glass, and most complicated and 
costly arrangements. Prof. Pritchard of Oxford, however, has 
found that this little instrument (which is only a special form 
of micrometer capable of attachment to any equatorial telescope 
at a cost of about 25th that of a Heliometer) can be utilized, and 
indeed, I may now say has been utilized to do work which will 
in every way compete with the best work done by its costly 
rival. 
The construction of this instrument is very simple, and will 
be easily understood by reference to the annexed Figure, which 
is a perspective view of the instrument. 
A plate of glass about 24 inches square is ruled with 21 lines in 
one direction ;4 inch apart, and 2 lines in the other direction 2 
inches apart. The extreme lines of the set therefore form a per- 
fect square of 2 inches. 
These lines are ruled with exceeding accuracy and care, but 
provision is left for ascertaining any errors that remain either as 
to distance or want of perfect squareness. 
Along one side of the square is mounted a micrometer frame 
in the ordinary way, actuated by a screw of 100 threads to the 
inch, this micrometer frame carries eleven lines corresponding 
exactly to each alternate line in the glass reticule, so that when 
the first spider line is made coincident with the first diamond 
line on the glass, the last spider line will be coincident with the 
last line on the glass, and each of the spider lines will be 
coincident with all the odd numbers of diamond lines 1, 3, 5, 7, 9, 
11, 18, 15, 17, 19, 21. Over this glass plate is placed a brass cap 
