602 HANDBOOK OF PHOTOGRAPHY 



if a genuine seal is available for comparison. Adhesives and binding materials may 

 be identified or compared by the fluorescence method. 



Works of Art. — Ultraviolet photography is an important instrument for the deter- 

 mination of the authenticity of works of art and for the detection of repairs, over- 

 paintings, and forged signatures. In the field of sculpture, marble, alabaster, 

 limestone, sandstone, and granite can be examined by ultraviolet fluorescence to 

 determine the origin of the stone, its age, and the presence of restorations and rework- 

 ing. Precious and semiprecious stones can sometimes be distinguished from paste 

 stones and artificial pearls from the natural variety. Old ivory can be distinguished 

 from the more recent material and from bone and the artificially aged material from 

 that colored naturally. The method has also been successfully applied to the examina- 

 tion of textiles and ceramics, glass, enamels, prints and drawings, and particularly 

 paintings. In this case it is used in conjunction with chemical analysis and photog- 

 raphy by X rays and the infrared. Restorations may be distinguished by the differ- 

 ence in fluorescence between old and new paints and by changes in the fluorescence 

 of the varnish if the restoration is made over the varnish. Sometimes a varnish will 

 fluoresce to such an extent as to mask the fluorescence of the underlying pigments, and 

 allowance must be made for this. The use of the method in the examination 

 of palimpsests and other documents has been described in the preceding section. 



Other Applications. — It is impossible to enumerate all the uses to which ultraviolet 

 photography can be put, and the interested reader should consult the works listed at 

 the end of this chapter, particularly "Fluorescence Analysis in Ultra-violet Light," 

 by Radley and Grant. Important applications are in the field of photomicrography, 

 particularly photographing by the 3650 A. line of the mercury spectrum, which results 

 in increased resolving power over that obtainable by visible light. Fluorescence 

 photography can also be satisfactorily carried out through the microscope. These 

 subjects and the very important physical applications in spectrography and astronomy 

 are beyond the scope of this chapter, but there is a very extensive literature devoted 

 to them. 



Warning. — Attention should be drawn to the ease with which wrong conclusions 

 can be drawn from idtraviolet and fluorescence photographs. The photographer 

 will require considerable experience before he can use the method reliably, but when 

 once this has been acquired, the method is an extremely important one. It should be 

 remembered that many dyes are bleached by exposure to ultraviolet radiation and 

 that it is much used for artificially aging materials. Caution should, therefore, be 

 used in its employment. 



INFRARED PHOTOGRAPHY 



Infrared photography is by no means a new subject, since methods of making 

 photographic plates responding to the infrared were known to scientists during the last 

 century. The subject assu-med a new interest, however, in 1931 when infrared sensi- 

 tive plates began to be made which could be used with the ease of ordinary plates and 

 films. Within a few years it became possible to record the spectrum out to beyond 

 13,000 A. in the infrared, and many discoveries of great importance to physics and 

 astronomy resulted. At the same time the subject grew to be of great importance in 

 other fields of science, technology, medical and aerial photography, long-distance 

 photography, criminology and documentary' photography, cinematography, and the 

 commercial and amateur spheres. 



The usefulness of infrared photography is due to the discovery of classes of dyes by 

 addition of which photographic emulsions could be made to respond readily to wave- 

 lengths longer than those which could be recorded on panchromatic materials. The 



