REPORT FOR 1897-PART II. APPENDIX NO. 10.

729

•Captain Deville recommends the perspectometer to be made by first drawing it on paper in a fairly large scale, and tben inaking a négative of it, reduced i>hotographically to tbe desired size of tbe finished perspectometer. A positive copy may now be made on a transparency plate, whicb, if bleacbed in a solution of bickloride of mercury, will show white lines on clear glass. For the sake of better préservation such perspectometer, when completely dry and hard, should be var-nished.

When using the perspectometer for transferring figures from their perspectives to the ground plan, when such figures are situated in planes perpendicular to the picture plane but inclined against the horizon plane, the center of the perspectometer is placed upon the principal point P of the picture plane, the same as before, but the perspectometer is now revolved about P until the parallel lines of the same are parallel with the trace of the inclined (figure’s) plane on the picture plane. In this case the trapezoids of the perspectometer represent the perspective of a net of squares situated in the inclined plane, the squares of which are now to be projected into the ground plane.

This net of squares in the inclined plane, when projected into the ground plane, will be com-posed of rectangular figures of equal size, their long sides being in a direction at right angles to the picture trace (or ground line) and of a length equal to that which is intercepted between two adjoining radiais of the perspectometer on the trace of the inclined plane (on the picture plane), while the short sides of those rectangles (forming the projection in the ground plan of the squares in the inclined figure’s plane) will be equal to the lengths obtained on the ground line by project-ing the points of intersection of the radiais of the perspectometer with the inclined plane’s trace on the picture plane upon the ground line of the picture plane.

The construction of the rectangular net on the ground plan may now be made in an analogous manner to that mentioned for the squares, and the drawing in of the figure on the ground plan with reference to its position within the trapezoids of the perspectometer is accomplished in the usual manner.

Should the figures be situated in planes that are inclined to both the picture and the ground planes, then the figure is first projected upon a plane perpendicular to the picture plane, and having the same trace in the latter as the inclined plane.

VI. The perspeetograph.—Numerous instruments hâve been devised for drawing perspectives from plans or from nature, mechanically, or by means of optical devices, some of which may inversely become of use for transcribing perspectives of figures into orthogonal projections.

The perspectograpk, invented by H. Eitter, serves to construct the orthogonal projection of a plane figure from its perspective, or to draw the perspective from the plans of the object without referring to the object itself.

Bitter’s instrument, manufactured by O. Schrœder & Co., in Frankfort-on-the-Main, bas been patented in Germany, October 13, 1883, under No. 29002. It was devised primarily for architectural purposes.

This instrument in its présent form, composed largely of wood, is not well suited for surveying purposes, as it contains too many sources of error due to lost motion in its bearings, still, its theory being sound, there is no reason to question its ultimate value, even for précisé work, if it were carefully made by an expert mechanician (excluding the use of wood and using métal throughout), being guided in its construction by the demands of the greatest précision attainable. As a carefully constructed instrument based on the présent pattern may become useful in platting the data of a topographie reconnaissance where, in the nature of the work, rapidity in making the results practically available is of greater importance than a high degree of accuracy, the following description of this instrument may not be out of place here. For its methods of use in photo-topographie surveying we respectfully refer to Capt. E. Deville’s work on u Photographie Surveying” already mentioned.

Wehave seen (Chapter I) that the platted position of a point in the ground plan may be found from its perspective (in vertical plane) by locating the point of intersection of the horizontal projection of the ray: ustation—pictured point” with the line of direction itself. (The latter with its vertical plane is revolved about the trace of the vertical plane in the ground plane (as axis of rotation) into the ground plane in which plane the point of intersection is located.)

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Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 97,74 %.

La langue de reconnaissance de l'OCR est le Français.

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