289 



Tables for Correcting Errors in Camera Drawings and 



Photomic rogra phs. 



By Edward M. Nelson, F.R.M.S. 



{Read May loth, 1896.) 

 It is well known that in drawings made with a camera 

 distortion will be present unless the whole surface of the paper 

 be kept at a uniform distance from the centre of the eye lens ; 

 in other words, the picture to be accurate must be drawn in the 

 hollow of a spherical shell whose radius is the distance of the 

 centre of the eye lens.* As this process is not very practical 

 the following tables show the errors in drawings made on a 

 plane surface, the distance of the centre of the eye lens from 

 the paper being 10 inches, or 254 mm. By applying the correc- 

 tions given in the tables to any drawing made on a plane 

 surface an accurate picture can be very easily obtained. It 

 will be seen that the error is not so great as is generally sup- 

 posed, for it only amounts to 5 per cent, in an image which is 

 about 8"0 inches in diameter, or 4'0 inches on either side of the 

 vertical from the eye lens to the centre of the drawing. A 

 drawing intended for illustration in scientific works seldom 

 attains 5 inches in length ; the first table shows that the error 

 for that size is only 2 per cent. Thus, if the true magnification 

 were 100 diameters the distortion would correspond to a 

 magnification of 102 diameters. Therefore, in accurately 

 measuring magnifying powers the images of the micrometer 

 scale should never exceed 2| inches, or 62 mm. 



The first table gives the percentages of the errors with 

 various diameters of images ; the second table the actual 

 amount to be subtracted from a drawing made on a plane 

 surface to make it similar to one drawn in the hollow spherical 

 shell. The figures in the first and third columns of the second 

 table are radii, viz., the distances of the images from the point 

 where a perpendicular from the centre of the eye lens would meet 

 the paper ; the second and fourth columns show the amount to 

 be subtracted from the lengths in the first column to correct the 

 errors on one side of a drawing ; therefore in correcting the 

 whole drawing proper amounts must be taken from the other 

 sides as well. 



* An ordinary camera drawing on a plane surface, 10 inches from the 

 eye lens, is a gnomonic projection of a sphere 20 inches in diameter, hence 

 the distortion follows the law of that method of projection. 



