214 Principles of Photography. 



eludes the oblique and marginal rays, and admits only those 

 which fall about the middle of the lens ; these being the only 

 ones brought to the same focus. 



When a cylindrical pencil of rays strikes obliquely on a lens, 

 they describe upon its surface, not a circle, but an ellipse ; and 

 the focus of the rays at one or both extremities of the major 

 axis is farther from the lens than that of the rays at the ex- 

 tremities of the minor axis. Two false foci, therefore, are the 

 result ; and the defect in the lens, thus produced, is termed 

 astigmatism — a point in the object becomes a line in the image. 



In repose, the eye, by its natural rotatory movement, can 

 embrace a visual angle of from 70° to 80". Hence, a land- 

 scape should subtend an angle of at least 60° ; but ordinary 

 instruments cannot, without distortion of the lines, embrace 

 one greater than about 30°. Many attempts have been made to 

 overcome this difficulty ; the most successful of the expedients 

 proposed, seems to consist in the use of two acromatic menisci, 

 placed opposite to each other, and at such a distance that their 

 convex surfaces, which are external, may form portions of the 

 same imaginary sphere. Between them is a diaphragm, having 

 an aperture which, by a simple contrivance, may be changed for 

 a greater or a less, and is always exactly in the centre of the 

 sphere. With such a combination, the angle is enlarged to at 

 least 75° ; and the picture obtained is so free from distortion 

 that if the original is a flat surface, and of the same size as the 

 picture, when one is superimposed on the other, their details 

 will exactly coincide. Such is the extent of angle thus ob- 

 tained, that the whole Hotel de Yille, of Ghent, was taken 

 with a globe lens of this kind, at the distance of twenty-eight 

 metres ; and such the depth of focus, that distant objects were 

 found in as good focus as those which were near. Its defect 

 is, that spherical aberration does not allow it to be used with all 

 apertures [Repertoire Encyclopedique, Dec. 1863]. 



Spherical aberration causes different images to be formed 

 by the rays at different distances from the axis of the lens, so 

 as to produce a luminous zone round its principal focus. It is 

 directly proportional to the size of the lens ; but is diminished, 

 or even destroyed, by an eliptical or hyperbolic curvature — 

 which, however, in practice, it is very difficult, if not impossible 

 to produce — and by diaphragms. Hence the proper use of the 

 latter is of the highest importance to the photographer ; the 

 diaphragm is the pupil, which, by its contraction or dilatation, 

 regulates the quantity of light that enters the camera — the 

 photographic eye, intercepting all useless or mischievous rays. 

 Not only the size of the aperture, but the position of the 

 diaphragm is of moment. It should be in front of a single 

 lens, and between the lenses, when two are used ; its distance 



