MANUFACTURES.
27 J
Manufac- microscope a must be moved to or from the centre oftures. circle, as the microscope is found to measure too
v-' small or too great a space; for-, the original divisions
bein°- radiating, the nearer the microscope is moved tothe centre of the circle, the smaller will be the linearvalue of an original space, and vice versa. Theseadjustments must be carefully performed, and verifiedat intervals, until the angle moved througn by the bar s,governed by the arc c, coincides accurately in the mi-croscope a with an original space, and the middle divi-sions of the arc and original divisions agree with thecross and hole in the microscopes at the same time.Process of (328.) The apparatus is now ready to cut divisions : tocutting the per f orm which bring the micrometer head of the miero-ivisions. £ e a tQ zero . an d let the cross in the microscope bbe bisected by an extreme division on the arc c; thenbrinn- the zero original division to bisect the hole in a.It wTll be recollected, that the zero original division wasplaced half an original space from the real zero; andwhen the apparatus is placed as above directed, thecutting point will be found half an original space fromthe original division, so that the zero division of thecircle will be in its right position The microscopesand circle being in this position, let a division be cut,then move the bar « by the pushing screw y, until thenext division on the arc bisects the cross ; then movethe circle by its tangent screw until the original divisionbisects the hole, and cut again. Proceed in this mannerthrough the whole original space; then leaving thecircle in the position in which the last division was cut,move the bar back to the first division on the arc again,and observe that the next original division now bisectsthe hole in a; then proceed as before to cut the wholeof the divisions in that space, and the same for thewhole of the circle. It may be observed, that a test ofthe permanence of the adjustments of the apparatus isformed at every space by the original divisions bisectingthe hole in a, when the microscope b is moved back to
the beginning of the arc. , . ,
Thetonghandlex, fig- 3 and 4, which moves the cuttingpoint, prevents any partial expansion in the apparatus;and for this purpose the whole may be enclosed andviewed through a plate of glass at the top. A narrowslit will be sufficient for the handle to pass through ;the handle a serving the same purpose as the connect-ing bar 25 in the engine, plate xxvi. fig. 1, and is joinedlike that to the vertical slide 34 of the moving appara-tus. But there is an arm 2 attached to* which projectsunder the frame e to lift the cutter off the work, whenthe bar x lowers ; therefore the action of the lever 22,plate xxv fie 1, will be reversed when the point iscutting the divisions, by which arrangement the barx will be raised while making the cut, and lowered toreturn.
The best light is considered to be an Argand lamp,placed outside the apartment in which the operation isperformed, and refracted or reflected through a hole tothe divisions. This will likewise give sufficient light inthe room to move the adjusting screws and cut thedivisions. These arrangements prevent the possibility ofpartial expansion from the heat of the light, and en-closing the apparatus, prevents the ■same from theproximity of the body, the breath, or the heat of thehand ; and as this is the only thing that can affect theaccuracy of the operation in this method of dividing, itmust be guarded against in every direction, which theconstruction of the apparatus greatly facilitates. It
may here be noticed, that the effect of partial expansion Machinery,must be observed in the second part of the process, v—>when the apparent error of the large arcs is taken.
For this purpose there should be several thermometersapplied at various parts of the circle; and before observing,it should be ascertained that they all indicate the sametemperature. This is of the greatest importance, as theerrors obtained by trisection, if this is not scrupulouslyattended to, will not agree with those obtained bybisection ; but in the smaller arcs it is only necessarythat they individually should be at the same tempera-ture. The microscopes used have the focus of the objectglass about ten times the distance of the plane of thedivisions from it, and the screw has 100 threads in aninch; this is moved by a micrometer head having 100divisions, so that by this arrangement the value of eachdivision is totot °f an inch. The cross wires areplaced at the angle 20°, and, with a little experienceand fine optical powers, the bisection of the cross by adivision may be observed within this value.
The method of filling up an original space by copyingan accurately divided arc by microscopic means, maybe applied in various ways. First by a frame rollingon the arc of the circle carrying the microscopes, andthe cutting frame and arc to be copied on the stand.
The microscopes may also be attached to a bar havinga Y at one end bearing on a cylinder on the centreof the circle ; but these modes are objectionable, as theyencumber the circle to be divided.
(329.) This method of dividing can with facility heap- Linearplied to linear instruments. For this purpose a plane of dividing,brass, or cast iron, about eight inches wide, and of the re-quisite length, having its edges perfectly straight,,is neces-sary to support the apparatus, which in this case is to movealong the bearer or plane, being guided in its motion bystuds rubbing against the edge and kept in contact by aspring and rollers acting on the other edge. The seriesforming the original divisions (supposing a scale of inches
and parts) is^= 12in.^-= 6 in.-^ = 3 in., the errors
of which are to be taken and computed as directed forcircular dividing; and these can be verified by bisection
The
36 IB 9
down to 4.5: for — = 18 in. — = 9 in. - = 4.5 in.
2 2 2
space of 1.5 being filled up with equal and parallel divi-sions, the angles on the arc can be obtained by makingthe difference of the tangents to the radius c M, plate xxv.fig. 11, equal; for suppose the scale to be divided intotenths of an inch, then the first tangent from the middle ol
, 1.5
the arc = —— in. =.75 in.
A
tangent of the extreme division on the arc will = ‘—>
cM
and the tabulated natural tangent of the other angleswill .'»■- • ° ■
and the tabulated natural.75
sca l e _J_. The original divisions are to be cut i
M
in their
true places, precisely as described in circular dividing;and in cutting the whole of the divisions, the onlydifference is, that the apparatus is movable instead ofthe instrument to be divided.
By this system accurately divided scales of any de-scription can be obtained; as it appears peculiarly2 n 2