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UNITED STATES COAST AND GEODETIC SUEYEY.

Centrolineads are usually sold in pairs 5 one serves to work on the left side of the principal point and the other on the right side.

V. The perspectometer (as used by Capt. E. Deville).—The perspectometer is used to dispense with the construction of the squares on the perspective when applyiug the “method of squares” (Ohapter I, Paragraph IX) to draw a figure in the ground plan by means of its perspective.

On a thin, transparent film (glass, xylonite, isinglass, horn, etc.,) two parallel lines AB and DD', fig. 121, are drawn intersecting the common perpendicular pP. Make DP=PD' =pA=pB= distance line (focal length) and froinp lay off on AB (to both sides oip) equal distances:

pm = mn = no...................=pm'—m'n'—n'o/=....................

Join these points of division to P and draw lines through the corresponding intersections of the

radiais froin P with the perpendiculars AD and BD1, rr1, tt'...............which lines will

be parallel with AB and DD'.

The use of the perspectometer. —The perspectometer is placed upon a perspective with P on the principal point and DD' coinciding with the horizon line. The ground line of the perspective may fall in XI, fig. 121, it will be divided into equal parts by the radiais froin P, and the trape-

zoids of the perspectometer represent the perspectives of the squares in the ground plane having the equal parts on XY as sides.

By placing the perspectometer on the perspective in the manner indicated above the squares covering the perspective of the figure that is to be platted iconometrically on the ground plan are at once apparent, and only those re-quired for the drawing of the figure in question are drawn on the ground plan.

The sides of the squares to be drawn on the ground plan (their side lengths are equal to the divisions on the ground line between the radiais drawn from P) are laid off from the trace of the principal plane on the ground line, and the position of the front line nearest the picture trace (or ground line) is laid off on the ground plan either by estimation or construction. The estimation of the position of this line (corresponding to tt') on the ground plan is made by noting the fraction of a square’s side which represents the distance (between tt' and XY, fig. 121) from the ground line on the perspective.

The same perspectometer serves only for perspectives which hâve the same distance line (like photographs of distant objects taken with the same lens), different distance lines requiring different perspectometers.

The width^ P should be equal to the height of the horizon line above the foot of the picture; the radiais from P need not extend beyoüd the width of the picture, the distance points D and D1 having been taken as the limit of the perspectometer in the figure (121) merely to show more fully the principles involved in its construction.

The length of a single division on the line AB should be selected with reference to the resulting equal division lengthsof the lowest ground line usedfor the pictures, as the dimensions of thedivision lengths on the latter give the measure for the sides of the squares to be drawn on the ground plan.

These division lengths 011 the ground line should be in harmony with the scale of the plan and with the degree of accuracy that may be required for the délinéation of the topographie features. The smaller the size of the squares is selected on the ground plan the more accurately the transfer of the figure from its perspective to the ground plan may be made, the same principles being involved in this method of iconometric platting as in the well-known method of reducing drawings by means of two sets of (hair) squares, the ratio of their sides corresponding to the scale of the required réduction.

d p D

Fig.IZI

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