174 An Account of Experiments for determining the Length of 
focus, the distance from the object-glass to the spider’s threads 
3:25 inches, the focus of the compound eye-glass rather less than 
one inch, the magnifying power 18 times. In the other micro- 
scope, which I shall call. the fixed microscope, the olject-glass 
is of three-quarters of an inch focus, and the magnifying power 
consequently greater. The micrometer head is divided into one 
hundred parts. 
Each microscope slides in a tube, which is fixed in a plate of 
brass forming part of its support; and this plate moves in a dove- 
tail, by which the microscope may be brought over the object 
to be viewed, when it is firmly clamped by a screw. Aue 
A piece of .well-seasoned mahogany, four inches and three- 
quarters, by three inches, served as a’beam to which the sup- 
ports of the microscopes were screwed, their centres being 39:4 
inches asunder, 68 ¥ 
Two screws with milled Leads supported the extremities of 
the beam in front, and a piece projecting from the middle of the 
beam behind served as a third leg. By means of the screws, 
the focus of either microscope could be nicely adjusted at plea- 
sure, without any risk of altering their distance from each other. 
My first object was to ascertain the degree of precision of 
which vision is capable when assisted by the microscope. For this 
purpose, a very fine line was drawn on a polished piece of brass ; 
and the microscope being carefully adjusted so as to be free from~ 
parallax, by causing the image of the line to bisect the angles 
formed by the spider’s threads, moving the eye to the right and 
left and remarking whether the image changed its situation, and 
if it did, varying the distance of the microscope from the object 
accordingly, until the line appeared stationary, the micrometer 
screw was turned back, and the spider’s threads brought. up 
again till the angle formed. by them appeared to be accurately 
bisected by the line. The division of the micrometer was then 
noted, and this was repeated several times with scarcely a sensi- 
ble difference in the result; and thus I assured myself that no 
error worthy of remark was to be apprehended from imperfection 
of vision. 
The next step was to determine the value of one-division of 
the micrometer head. By the kind interest of Sir Joseph Banks, 
I was favoured with the use of the standard scale which belonged 
to the late Sir George Shuckburgh Evelyn, and which is de- 
scribed in the Philosophical Transactions for 1798. This scale, 
the work of Mr. Troughton, is second to none in the kingdom 
in point of accuracy of division, and is too well known to render 
any further remark necessary. The microscope being carefully 
adjusted for parallax, one inch, from the 39th to the 40th, was 
measured by successive tenths, and the mean taken as the value 
of 
