( ) 
Slider is of an equal Thicknefs with the- Side of the 
Tube, and has a Groove, G G, F/^. i. cut for it in that 
Side, parallel to the /^xis, and long enough to give 
room for its Motion, to fet the two Specula at the diffe- 
rent Dilfances, which the feveral Eye-Glafles require. 
It. refts on the Infide againft two thin Ledges, faftened 
within the Tube along the Sides of the Groove. On 
the Outfide it is kept in its Place by a Aiding Shutter, 
not expreffed in the Figure. In the Middle it has a 
Cyliiidrick Cavity, D; Fig. 2. whofe Axis is exadly 
perpendicular to its inner and outer Surfaces. Each of 
the Boxes, in which the Eye-GlalTes are contained, is 
fitted to this Cavity. The beformention’d Brafs Arm 
is fix’d into the Infide of this Slider, towards the End 
fartheft from the Objed-Metal ; it rifes perpendicular 
for about two Inches, and is made flat, fo as to turn one 
Edge to the Rays, which come from the Objed. About 
it is bent forwards and flatted the other Way, fo 
that when the Back of the oval Plane is held flat to it; 
by the two Screws c c, the Axis of the Cyliiidrick 
Cavity may fall on the Centre of its forefide, inclin’d 
to its Surface in an Angle of fomething lefs than fourty 
five Degrees. This Angle is brought to be exad by 
two very final! Screws, / i, whofe Threads take hold 
in the flatted End of the Brafs Arm, and their Points 
bearing againft the Back of the Oval, raife one End of it- 
a little from the flat of the Arm. ‘ The Specula are fet 
at their due Diftance, by turning of a long Screw, C C, . 
for which there is a Nut lodged in the Slider at^ *, the, 
Screw is- kept from moving backward or forward, when 
’tis turned, by a Brafs Plate,- F, which is to be fix’d to 
the flat End of the Side of the Tube, and.taken off at 
Pleafure. Each of the Eye-Glafs Boxes^ H, has a 
Screw on the outward End, to faften to it a . Bowl, or 
Difh 
