678

UNITED STATES COAST AND GEODETIC SURYEY.

Prussia) of Dr. Meydenbanr?s surveying caméra commands an angle of about 100°. By exclnding an extern al ring of the effective disk of tbese lenses by means of a diaphragm, pictures are obtained subtending an angle of but 66°, requiring six plates for a complété panorama.

This caméra bas neither telescope nor vertical circle but it is provided with a horizontal circle, thus enabling the operator to control the révolutions of the caméra in azimutb.

After this caméra bas been set up and adjusted over a station the panorama is pbotograpbed by exposing six plates in succession, eacb successive turn in azimutb of the caméra covering an angle of 60°, flg. 53, and two adjoining plates lapping over eacb other by 3° in arc. Tbese com-

mon margins (like Paganinfs plates) contain identical sections of the panorama view. Tbey may serve to find the value for the focal lengtb of the pictures, and tbey control the permanency of the camera’s adjustmentsdur-ing one complété révolution in azimutb.

(a) Détermination of the focal length for the panorama views.—From the six plates, covering the entire horizon from one station, objècts may be selected on the center lines of the common margins of adjoining plates which sbould be équidistant from the principal lines of the two plates.

After having selected a sériés of sucb reciprocal points (using a magnifying glass if necessary) on ail six plates, we will bave obtained twelve déterminations, represented by the lengtb l, for the position of the principal line. The greatest discrepancy between any two values should not exceed 0’2 mm, if the instrument was well adjusted. The sum =21 of two sucb distances (between two of the corrected principal lines) will rep-resent the effective lengtbs of one picture, or the lengtb of one side of a regular hexagon, witb an inscribed circle of the radius equal to the constant focal lengtbs (=/) of the négatives.

This length =/may be found grapbically or it may be computed from the formula:

J tan 30° •

When positive prints are to be used in the iconometric map construction it will become necessary to correct this focal lengtb / to correspond with any changes tbat may bave taken place in the dimensions of the prints when compared witb their négatives. By comparing the distances between the “ teetb ” (marking the principal and horizon lines) on the négative witb those included between their contact prints on the positive the total linear changes of the print in the directions of the principal and horizon lines are readily found.

We bave witb reference to flg. 54 : ab — original lengtb included between the teetb marking the horizon line on the négative, a'b' = lengtb of horizon line (included between the pictured teetb) on the positive print. co = f = constant focal lengtb of caméra or négative.

If we draw the triangle abO, place the line a'b'

(measured on the print) parallel witb ab and move the same (maintaining its direction parallel witb ab) toward (or.from) O until a' falls upon ao and b' upon bo, then c'O will be the focal lengtb of the photograpb (“contracted,” in our case). This détermination of /sbould be made for every print that is to be used in the iconometric map construction.

The topographie map is grapbically constructed from the négatives and prints in a manner very similar to tbat described for Colonel Laussedat’s metbod.

(6) General method of iconometric platting.—Witb reference to fig. 55 we bave:

I and II = two négatives of plates exposed from caméra stations I and II, respectively.

III = baseline, measured in the field.

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