406 On the Meridian Instruments of the Dudley Observatory. 
The circles are three feet in diameter, and entirely protected 
on the outer side by the piers. They are of the form which long 
experience has recommended to Messrs. Pistor and Martins as 
the best,—not too heavy at the rim, and with radial arms thick- 
ening in both dimensions towards the centre. The screen-tubes 
for the microscopes draw back automatically as soon as the coun- 
terpoises are relieved of a portion of their burden, and it is thus 
possible to have them very close to the circles, when in use, 
' without incurring any danger of injuring the graduation when 
the instrument is lifted for reversal. 
e eye-piece has a vertical as well as a horizontal motion; 
and the diaphragm, which is of course adapted for chronographic 
observation, is provided with both a horizontal and a vertical 
micrometer,—the former being especially intended for the obser- 
vation of polar stars, according to the method recently adopted 
in the Paris Observatory, and which Professor Bache had inves- 
tigated in 1849. 
The method of Hansen for measuring and eliminating the 
effect of flexure comes from authority too high, and commends 
itself too strongly, to justify us in lightly setting it aside. But 
advantages entirely incompatible with its employment presented 
themselves in such number as to induce me to accede to the 
earnest recommendation of the artist, and abandon the original 
bony of interchangeable eye-piece and object-glass. Some of the 
connection of these from their gearing or screw-heads. Indeed, 
nothing like the former can be reasonably demanded,—a sacrifice 
which seems disproportionate to the end to be attained. More 
over, the new pets solace rel equipped with more than a usual 
amount of internal mechanism, although the arrangement and 
support of this latter has been planned with an especial view to 
