( 3°4 ) 
Metal is placed fo, as to have its Axis coincide with that 
of the Tube, by the means of three fmall Buttons fix’d 
to the Infide of the Tube, having their hinder Ends all 
in the fame Plane, to which this Axis is perpendicu- 
lar. Two of thefe appear at a a, the third being at the 
middle of the Bottom of the Tube, is not feen. The 
forefide of the Metal refts againft thefe Buttons in three 
Points of its Circumference, nearly equidiftant from 
each other, and is held to them by three Screws, (one 
of which appears at which run through the odaii- 
gular Board at the End of the Tube, and bear againft 
the Back of the Metal, (in three Points, whicli diredly 
anfwer thofe three on the forefide) with juft fo much 
Force, as is requifite to keep it fteady in its Place. They 
muft not be fcrew’d harder againft the Metal for Fear 
of bending it, which (tho’ it is half an Inch in Thick- 
nefs) a very little Force is fufficientto do. When the 
Inftrument is not ufed, thefe Screws are loofen’d, and 
the Objed-Metal is taken out and laid by, to prevent 
its tarnilhing. 
The oval Plane is compos’d of a Plate of the fame 
Metal with the great Speculum, about rf or of an 
Inch in Thicknefs, folder’d on the Back to another 
ofBrafs. Its Breadth is fomething lefs than half an 
• Inch, and is in Proportion to its Length as i to v' 2. At 
one End of the Oval, the Brafs Plate projeds a little be- 
yond the other, and has a Screw cut through it in that 
Part, as likewife another diredly againft the Centre of 
the forefide. The other End is cypher’d away on the 
Backfide, that it may intercept as few of the Rays, in 
their PalTage towards the Objed-Metal, as is poflible. 
The two Screw-holes in the Back ferve to fix this Oval 
A, fig, 2. to a Brafs Arm, B, which is faftened at 
the other End into a Slider EE. Bg* i. and 2. This 
Slider 
