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THE POPULAR SCIENCE MONTHLY. 



sides, as in the diagrams A, B, C (Fig. 1), though with a feebler 

 outline. A picture thrown off" quite out of drawing by such a lens, in 

 which straight lines turn out as curves, is evidently inaccurate. The 

 inaccuracy may not be felt by many, but it exists. It may perhaps 

 be expected that this defect disappears in the case of what are called 

 correct lenses, but let the attempt be made to obtain a view with 

 these correct lenses of lofty buildings taken from a low position. The 

 lines that ought to be perpendicular commonly converge upward. 

 This is caused by the photographer being obliged to direct his instru- 

 ment at an acute angle upward, in order to be able to take in a view 

 of the whole building. In doing this, perpendicular lines project 

 themselves, converging upward. To avoid this defect, lenses have 

 been made w r ith a very large field of view. These are called panto- 

 scopes. But these reproduce distant objects apparently on a very 



Fio. 2. 



small scale, and objects near at hand on a very large scale peculiar- 

 ities unnoticed by unprofessional persons, but detected by close ob- 

 servers of Nature. 



A remarkable phenomenon, exciting the wonder of the uninitiated, 

 is the distortion of spheres in photography. Let the reader imagine 

 a row of cannon-balls ; these will always appear balls to us, and the 

 artist will always draw them as a circle. But, if they are taken 

 through a lens with a large field of view, the balls situated near the 

 rim of the lens no longer appear circular, but elliptical. 



To explain this phenomenon, we must attend once more to the 

 mode in which the picture is produced. Let it be conceived that 

 there are three balls, A B C, in front of a camera, IT, with the lens o 

 (Fig. 2). Each ball projects a cone of rays on the optical centre of 

 the lens. This is continued within the camera, and cuts the surface 

 of the picture, if its axis falls obliquely upon it, in the form of an 

 ellipse, such as A G. Only, if the axis of the cone of rays is perpen- 



