134 Mr. Wollaston's Description 
distance of the nearest division from a fixed point. It occurred 
to me immediately, that this was the thing I wanted : be- 
cause a circle attached to the telescope of a transit instru- 
ment, and passing in review before such a microscope, or a 
pair of such microscopes, would answer the purpose. I did 
not then know, that a microscope of that kind had been ap- 
plied by the late Due de Chaulnes, to his dividing engine, 
for determining the divisions ; described minutely by him, 
and published in 1768 ; a copy of which is in our library. 
Neither did I then know of the same idea having been the 
foundation of Roemer's method of reading off the divisions 
on his Circulus meridionalis ; an account of which was pub- 
lished by Horrebow, in the beginning of this century ; 
where a reticule of ten squares was made, by trials of its 
distance from the limb of the instrument, to coincide with a 
division of ten minutes on that limb. With them I was not 
acquainted, till after my instrument was already in some for- 
wardness. Whether Mr. Ramsden took the first hint from 
either of them, and improved upon it, I cannot say. He has 
brought it into use among us : I certainly derived it from 
him ; and to him I acknowledge myself indebted for it. 
This method of reading off’ has, indeed, been applied al- 
ready with great success to different instruments ; but I do 
not know that it has ever yet been adapted to the transit. 
Circles of various kinds have been constructed with wonder- 
ful accuracy, yet all have been formed with another view; 
and their turning freely in azimuth, seemed to render them 
less fit for the purpose which I wanted ; i. e. a circle, firmly 
fixed, and turning truly in the plane of the meridian by 
means of a transverse axis ; with all the adjustments of a 
