50 Capt. Rater’s experiments for determining the 
In the other microscope, which I shall call the fixed micro- 
scope, the object glass is of three quarters of an inch focus, 
and the magnifying power consequently greater. The mi- 
crometer 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 dovetail, by which the microscope may be brought over 
the object to be viewed, when it is firmly clamped by a 
screw. 
A piece of 'well seasoned mahogany, four inches and three 
quarters, by three inches, served as a beam to which the 
supports of thejnicroscopes were screwed, their centres being 
39, 4 inches asunder. 
Two screws with milled heads, 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 ad- 
justed at pleasure, 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 
