23 



the bisection of the star itself, wliich is often no very precise opera- 

 tion. This llierefore decided me to adopting a mural circle resembling 

 those of Greenwich, except in the mode of supporting its axis, and 

 the number of microscopes : it is not well adapted to act as a 

 transit, but when there are two observers this is unnecessary; and the 

 removal of half the axis gives great facility to observing by reflec- 

 tion when that is thought necessary, or of using the vertical col- 

 limator.* My chief embarrassment was in deciding on the number 

 of microscopes, and here I was obliged to determine for myself upon 

 the investigations, which I have the honour of laying before the 

 Academy. 



Before the invention of circular instruments, we may wonder how 

 observations could be made with the least pretensions to accuracy. 

 Errors of division, accumulating through the whole limb of a 

 quadrant, and unchecked by any accurate determination of the total 

 arc, along with defects of the centre work, must have been present 

 in those instances when certain Italian and German astronomers 

 were found to be mistaken in their latitude 20 or 30 seconds, and 

 from which a celebrated astronomer was led to conclude that they 

 could never have made the observations which they reported. Rams- 

 den, though not the first who constructed an entire circle, was the first 

 who applied to it several readings, which, witii his invention of the 

 micrometer microscope, constitute an aera in astronomy. The circle 

 of Palermo, in common with the otliers, executed' by their great artist, 



and viewing an inclined collimator, would probably be the best portable instrument that has been 

 devised. 



* If the sun were observed by reflection from a surface of ink, it would obviate all heating of 

 of the instrument. As to the vertical collimator, I do not know that it has been tried ; by theory 

 its stability is to that of the common one of equal weight as 1 : jr. Ten grains placed at the ex- 

 tremity of my collimator, incline it 2." 



