g8 Sir George Shuckburgh's Account 
From inspection of the preceding table of observations it 
will readily appear, that I was much mistaken in my ex- 
pectation of an eccentricity of 8 or 10", for that, in truth, 
there seemed to be no fixed cause of error ; and that therefore 
the error of the centre had little to do in occasioning those 
differences in the opposite microscopes, which only once 
amounted to 4", and this in fact was double the error of 
the centre added to the sum of the errors in the two op- 
posite divisions, together with the error of twice reading 
off ; and that the simple error never exceeded, and but once 
amounted to i",6. This being the case, I think it fair to 
conclude, that the eccentricity never amounted to any sen- 
sible or measurable quantity, viz. never exceeded 1",* and 
that consequently all the variety we see, in the west or oppo- 
site microscope, arose from the error of the divisions, ine- 
quality of the points, imperfection in reading off, or a little 
play in the screw of the micrometer. But, as all these toge- 
ther never but once amounted to 2", I think it may fairly be 
presumed that that is the greatest error that will arise, in any 
observation made with this circle, when only one mi- 
croscope is used, and that probably only half that error will 
take place. 
* How extremely small a quantity this is, may be seen by considering that, on a 
radius of two feet, an arc 
of 10' amounts only to - - - 0,0698 inch. 
of 1' - - - 0,0070 
of 1" to { — in round numbers to about inch.) 0,0001 16 
Viz. to about 8 times less than the minimum visibile to the naked eye. This I 
reckon, with my own eye, at 8| inches distance, is about inch ; but then it must 
be considered that the microscopes magnify 16 times, and will therefore render a space 
visible that is inch, or about = o"f, which we shall soon find to be the fact. 
