598 HANDBOOK OF PHOTOGRAPHY 



employed in the form of optically worked sheets over the lens, and the Corning glass 

 Violet Ultra No. 586, which is available in the form of molded and polished squares 

 for use over the source and which can be optically worked for use over the lens. Some 

 other Corning glasses are suitable provided a certain amount of transmission in the 

 red can be tolerated. They are the glasses: Heat Resisting Red Purple Ultra No. 587; 

 Red Purple Ultra No. 597; Red Ultra No. 584; and Red Purple Corex A No. 986. An 

 ultraviolet-transmitting glass made in England by Chance Bros, and Co., Ltd., is 

 known as No. 14 Ultraviolet, while the Jena Glass Works of Schott und Gen, in Ger- 

 many, make similar glasses known as UGl, UG2, and UG4. All these filters transmit 

 a relativel}^ narrow band of ultraviolet, extending roughly from 3000 to 4000 A., with 

 a maximum at about 3600 A. They are therefore very suited for isolating the 3650 A. 

 line of the mercury spectrum. The General Electric Vapor Lamp Co. manufactures 

 Cooper-Hewitt lamps in which the tube is made of a visually opaque, ultraviolet- 

 transmitting glass known as Nico. Manj^ other filters transmitting ultraviolet and 

 absorbing visible light have been described, but in general they are not so convenient 

 or efficient as the glass filters. Perhaps the most interesting is a plate of quartz or 

 ultraviolet-transmitting glass coated with silver, \yhich has a transmission band at 

 3200 k. 



Filters of the second kind, absorbing ultraviolet radiation, and transmitting the 

 visible freely, are used in fluorescent light photography on the lens of the camera. The 

 most used of these is the Wratten No. 2A filter, which absorbs all radiation of wave- 

 length shorter than 4100 A., and transmits the visible fully. The Wratten No. 2 

 Aesculine filter absorbs below 3900 A. but is fluorescent, and the No. 2A is to be pre- 

 ferred. Many types of liquid filter have been proposed for this purpose, but some of 

 those which are often recommended are not verj^ efficient absorbers. A layer of a 

 solution of cerium ammonium nitrate, 1 cm. thick and containing sulphuric acid to 

 dissolve the salt in a concentration of 0.1 per cent, absorbs below 3750 A. and in 1 per 

 cent concentration absorbs below 4600 A. A layer of triphenylmethane solution (0.5 

 g. in 75 cc. cf ethyl alcohol) 1 cm. thick, a filter often recommended, is not satisfac- 

 tory since it transmits freely above 3000 A. 



Cameras. — Any camera employed for normal photograph}^ is satisfactory for use 

 in the ultraviolet by either method. For the fluorescent-light method the normal lens 

 may be used, as the photograph is made by visible light. The ordinary lens can also 

 be employed for the reflected ultraviolet-light method, provided it transmits the 

 wavelengths it is desired to record. For wavelengths shorter than about 3500 A. it is 

 necessarj^ to use a quartz lens, and, if records are required below 2000 A., the lens must 

 he made of fluorite. However, photography in this short-wavelength region is only 

 possible if the air is evacuated from the camera and if plates containing little gelatin 

 are used. It falls, therefore, outside the sphere of operation of the ordinary photog- 

 rapher and is chiefly of interest to the physicist recording spectra. The focus of a lens 

 in the ultraviolet is not quite the same as in the visible region, but little difficulty is 

 encountered on this score if a small aperture is used. 



Photographic Plates and Films. — All plates and films are sensitive in the near 

 ultraviolet to about 2000 A., although on account of the absorption by the gelatin 

 their sensitivity begins to fall off at 2800 A. and is noticeably reduced at 2500 A. In 

 the region from 2500 to 3900 A., however, and in the visible region, normal photo- 

 graphic materials may be used. The plate or film selected will depend on the speed 

 and contrast desired, the choice being made in the same manner as for photography by 

 visible light, except that it should be borne in mind that the contrast in the idtraviolet 

 is lower than in the visible, and plates and films of high contrast are preferred. For 

 photography by the reflected ultraviolet method the highest sensitiAnty is given by 

 the uoncolor-sensitized plates which have the highest speed to visible blue light, such 



