CHAP. XVII.] REDUCING PLANS BY PHOTOGRAPHY 377 



black print supplied. Blue being non-actinic, only those parts 

 that have been inked in will now appear. 



If a blue print is not available, a tracing of the original 

 drawing should be prepared, omitting details unsuitable to the 

 smaller scale to which the original is to be reduced, and regulat- 

 ing the size of the figures, lettering, etc., as described in the case 

 of the blue print. The tracing is then reduced by photography 

 to the required scale. 



The lenses of the present day have been brought to great Sources of 

 perfection, and experience shows that they produce no distor- ^^°^' 

 tion even near the edges of a large photograph where it is most 

 hkely to occur. The only sources of error lie in the possibility 

 of the board upon which the drawing is pinned for photographing 

 not being perfectly flat, or not being placed at right angles to 

 the axis of the lens. Both these points require attention on 

 the part of the photographer. By the aid of a few large squares 

 on the original drawing and similar squares on the reduced 

 photograph, drawn in the usual manner, an error due to any 

 cause is quickly detected. All reductions are tested in this 

 manner before being accepted. 



Reference to plates A, B, C, T> will facilitate the preparation piates 

 of tracings and blue prints for reduction in scale in proportions iWustrat- 

 varying from one-half to one-fourteenth of the scale of the Prepara- 

 original drawing, intermediate proportions being regulated g^^g**^ 

 accordingly. From these plates a tolerably good idea can Prints for 

 be formed as to the amount of detail necessary to be in^gcair"^ 

 shown in each case, and other matters, such as the spacing by Photo- 

 of the soundings, size of the figures, lettering, thickness of ^^^ ^' 

 coast-line, etc. 



In the case of small reductions in scale it is unnecessary to Reduction 

 use photography. Large squares being dra^^^l on the original of Equal 

 document, and similar squares on the required scale on tracing- Squares, 

 paper, the details can be traced by fitting each square on the 

 tracing-paper over its corresponding square on the original. 

 The difference in scale being small, the several squares will 

 correspond so closely that no practical error is introduced. 

 The smaller the squares the more nearly will this be the case, 

 and their size should depend on the proportion that one scale 

 bears to the other ; the smaller the difference in scale the fewer 

 squares being required. 



