PHOTOGRAMMETRY. 601 



clearly seen in it ; the liability of error, judged of by tests applied 

 to my experiments, being about 1 per cent., viz., one chnin in one 

 and a quarter miles. It is evi'ient, therefore, that the camera 

 might be made valuable to the surveyor in furnishing him Avith 

 reliable details where required f(n- the compilation of topographical 

 maps, especially for parts difficult of access. Architects and 

 engineers might also find the application of photography upon 

 these lines convenient and safe in many cases where at present 

 they would not employ it, supposing that no reliance can be placed 

 upon the results. 



The writer is indebted to notes in the "Photographic Journal" 

 of June, July, and December, 1892, for suggestions upon the 

 subject ; also to a paper in the Transactions of the American 

 Mining Engineers for 189 ', by Messrs. Flipper and Noagles. 



XOTES AXD REMARKS UPON ESSENTIALS OF PHOTO- 

 GRAMMETRY. 



1. The focal length of lens used should be ascertained and 

 recorded on the back of every plate required for photogrammetrical 

 purposes. A permanent mark can be made on the camera at the 

 average focus, and the small difference from the mark observed 

 each time. 



2. Tlie camera should be placed level, that is, the sides should 

 be upright and the top horizontal. 



3. One horizontal and one vertical line should be drawn upon 

 the screen, and the lens be adjusted so that its optical axis 

 coincides with their jDoint of intersection; then the carriers should 

 have four studs corresponding precisely with the terminal points 

 of the lines on the screen which studs might be so arranged as to 

 be photographed upon the margin of the negative, thereby in- 

 dicating the exact position of the co-ordinating lines. 



4. When photographing a building measure the distance from 

 lens to a point on wall coinciding with the optical axis of the lens. 

 The position of the point should be noted in a book, with the 

 distance. 



5. Instead of tilting the camera it is best to fit the lens into a slot 

 so that it can be moved up or down an inch or two when required. 



6. All offset distances should be scaled from prints that have 

 not been toned or fixed, as they then perfectly agree Avith the 

 negative, but shrink or expand unevenly after wetting. 



7. The gelatine film of the negative, though it enlarges with 

 chemical treatment, affords when dry the same dimensions of the 

 luminous image it received when in the camera, the exact measure- 

 ment of which, taken occasionally upon the ground glass, will be 

 a convincing test of this. 



8. Celluloid fdms are manufactured in America for use instead 

 of glass, and afford a capital surface for working up field draughts 

 upon. 



