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in Optical Instruments. 21 
procured a plank of wood covered with paper of sufficient length, 
take the aperture of the object glass, and set it off at one end of 
the board, bisect it and draw a line at right angles to it, to the 
exact length of its focus: fix three strong needles into the three 
points of the focus and aperture of the object glass, and then 
stretch a fine thread over them, which will then represent the cone 
of light which forms the image; set off six or seven inches from 
the focal extremity, (an efficient stop cannot be placed nearer, 
without.contracting the field of view,) and ascertain the distance 
between the threads at this point, which will give the diameter of 
the fifth stop. Then divide the remainder of the focal length into 
five equal parts, (whatever it may be,) and the distance of the 
threads will give the diameter of four more stops, 4, 3, 2, and I, 
in the figure, all sufficiently correct for the purpose. The stops 
are then to be made and inserted into the tube in their proper 
places; it will not however be amiss to make No. 1, 2, 3, and 4, 
a little too large, and to confide the main business of stopping 
the false rays to No. 5*, which may be attached to the eye tube, 
and move along with it, in adjusting the focus of the magnifier. 
This will give the instrument the power of adjusting itself, to nearer 
objects on the earth, without losing any light from the effect of the 
stops, which otherwise must be adjusted to the shortest focus of 
the object glass, and supposed to act perfectly only with parallel 
rays. It will not be amiss to haye a very small eye-hole, placed 
correctly in the focus of the object glass, which will give a great 
facility of adjusting the stops, as it will shew by merely looking 
through the instrument, if they are correctly placed or not. Lastly, 
having fixed these, apply the eye-piece, carrying the lowest mag= 
* The way of regulating the aperture of this or any other stop to its situation 
in the course of the tube, will naturally be by pushing it up or down, till it 
strictly conforms itself to the size of the cone of rays at the point where it acts ; 
when once itis settled for the lowest magnifier,so that the image of its aperture, 
and that of the object glass correctly correspond, and shew no difference in 
measurement by the dynameter, the business is effected for all the ether 
powers, as they will always preserve the same relative proportion to each 
other, whatever may be the depth of the lens which is employed to form ay 
image of them, 
