Page 435 equipment and instruments 4854 



transferred to the map or chart by fitting the impression in place and burnishing the 

 detail. 



The setting for a pantograph to be used in making a reduction may be found from the formula, 



a 

 in which S is the setting for the three graduated arms, a is the distance, measured with any convenient scale, between any two points 

 on the original to be reduced, 6 is the distance between the same two points at the required reduction, measured with the same scale, 

 and / is the factor of the pantograph, which is 960 for most of those used by the Coast and Geodetic Survey. The use of the panto- 

 graph for reduction is limited to a ratio of about 1 to 20, or a setting on the arms of 48.0. For smaller ratios two reductions are required, 

 in which case the first should be made at a ratio of about 1 to 4 and the second at a ratio chosen so that the product of the two ratios 

 will be equal to the total reduction desired. 



4854. Projector 



A projector is an instrument by means of wliich the image of one survey may be 

 projected through a lens onto another survey sheet at a different scale. Projectors vary 

 from simple assemblies to elaborate commercially, manufactured instruments. They 

 are not generally used on board survey ships because the limited vertical space is not 

 sufficient and the projectors in common use require a firm foundation. They are 

 especially valuable in the Washington Office and in field plotting offices. 



The simplest projector is one in which the detail on a transparent sheet is projected 

 vertically downward through a bellows-and-lens assembly at varying scales. 



The projector in general use in the Coast and Geodetic Survey is designed so that 

 detail may be transferred, by means of a reflected image, from opaque as well as trans- 

 parent sheets. It consists of a stationary lower horizontal table and a movable upper 

 assembly containing a vertical table, the bellows and lens, the lights, and a front- 

 surface mirror inclined at an angle of 45° to both tables. The lens and bellows are 

 mounted under the light-tight box directly below the mirror so that the projected 

 image will appear approximately in the center of the lower table. The vertical 

 distance between the lower table and the upper assembly, and the distance between 

 the lens and the mirror, may be varied by suitable attachments. The arrangement 

 of the lens and the mirror with respect to the two tables results in the projection of a 

 direct image of the subject onto the lower table, thus permitting the ready transfer 

 of the detail. 



In use, the sheet containing the detail to be transferred is attached to the upper 

 table, and the sheet to which it is to be transferred is placed on the lower table. The 

 lower sheet is oriented to fit the upper sheet and the reflected image of the upper sheet is 

 brought to the scale of the lower one by means of a vertical movement of the upper as- 

 sembly and a separate vertical movement of the lens with reference to the mirror. The 

 correct setting is determined visually. When the projected detafl is in scale agreement 

 with the lower sheet and the projected image is in sharp and clear focus a perfect setting 

 has been attained. 



A projector should be tested occasionally to make certain that the plane of the- 

 upper table is accurately projected to the plane of the lower table. This is done by 

 placing a square grid on the upper table and projecting it to an identical square grid on 

 the lower table at a ratio of 1 to 1. If the projected image coincides on all sides, th& 

 projector is in adjustment. If perfect coincidence is not obtained, the upper table or 

 mirror should be adjusted by means of the attachments provided for that purpose. 



