<300 PROCEEDINGS OF SFXTION H. 



P 



The calculation is as follows : — Tan. angle A = ~ ; tan. angle 



A' = ^ 

 F 



(P) sealed from photo = 78; log. = 1-89209 



(F) focal length of lens = 850 ; log. = 2-92942 



A = 5-15', log. tan. 8-96267 



(P) scaled from photo, 190; log. 2-27875 

 (F) focal length 850 ; log. 2-92942 



A' = 12-36'; log. tan. 9-34933 



Having obtained both the angles, now substitute the measured 

 base 68 -7 5ft. for the focal length F, and find the two perpendicu- 

 lars, as follows : h = tan. A X D ; A' = tan. A X D. 

 (D) measured distance, 68-75: log. 1-83727 

 (A) calculated angle, 5-15'; log. tan. 8-96267 



h in feet =631; log-. 0-79994 



(D) measured distance, 68-75; log. 1-83727 

 (A') calculated angle, 12-36'; log. tan. 934933 



h' in feet = 15-36; log. 1-18660 



6-31 feet below axis of lens 

 15-36 feet above axis of lens 



21-67 feet, total height of telegrajjh post 



Or for brevity put h, and h' for heights of pnrtions of post, 



P \ 



then A = — X D 



F V 



p/ ( and h + h' =: height. 



A' = - X D 

 F ' 



In order to try horizontal angles with a proti-actor upon the 

 print it is necessary to produce the vertical line to a point below 

 the horizontal line exactly equal to the focal length, which in this 

 case is 8-475in., where they will be found to agree perfectly. This 

 important position may be called the " angle point." It is also 

 the point of station with regard to true geometrical perspective 

 of the whole picture. 



If the camera be moved about a quarter of a mile and the 

 distance measured, another photograph of the same view will 

 supply data for plotting with accuracy every object that can be 



