The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Crystalotype                                     Curvature of the Field
Crystalotype. Photography on glass was so called when first introduced, and the term is sometimes applied to a special photographic decoration of glass. (See Hyalography.)
Cubic Centimetre. A now de-legalised measure of capacity, but retained in the statutory cubic measures. By the Weights and Measures Act of 1878, a measure of capacity, called the "cubic centimetre,''was legalised; but under statutory powers conferred by the Weights and Measures (metric system) Act of 1897, the cubic centimetre has been de-legalised as a measure of capacity and a nearly corresponding measure, of capacity, the millilitre (one thousandth of a litre) has been substituted. In order to secure uniformity and accuracy, not only in measurement but in expression, it may be well for the public to insist on having graduates marked off in the now statutory millilitres rather than in cubic centimetres; although for the rougher photographic purposes the difference may be unimportant. Still, when an author expresses his intention in cubic centimetres, there is scarcely a justification for substituting millilitres. The uncer-tainty as to the litre and its subdivisions is quite an old trouble in scientific and technical laboratories. Thus Sutton in the second or 1871 edition of his " Volumetric Analysis," refers (p. 17) to two litres then in general use, and differing by 1*2 c c. Sutton rather illogically proposed to graduate instruments to a medium litre on the ground that "there are many thousands of graduated instruments already in use varying between these extremes, and as these cannot well be annihilated, the adoption of a mean will give a less probable amount of error." The state of things is perhaps worse at present than that years ago when Sutton wrote, but the new statutory definitions which have come into force since the last edition of this Dictionary was published should assist in securing uniformity and the rejection of all old and inaccurate measures graduated to the protean " cubic centimetre " of bygone days. (See Metric System.)
Curvature of the Field, or the Aberration of Form. The
image of a flat object should, to meet all the requirements of the photographer, be formed on a plane or flat surface; but, taking a single lens (fig. 38), we shall find that the focus for b is at e, whilst the focus for a is at g} for c at f h i represents the plane of the sensitive plate, from which it is obvious that,