796 HANDBOOK OF PHOTOGRAPHY 



These special ultraviolet-sensitive plates are now available through the Pkstman 

 Kodak Co. 



Gelatin absorbs light in the ultraviolet region of the spectrum, and the absorption 



becomes strong for wavelengths below 2800 A. 

 ^^--'"*?**^^-- Pi-actically all the light is absorbed below 2000 



• ' ' ' ■ -o A. even by very thin layers of gelatin. Shumann 



^ • Q o' o o ' ■ plates which contain only a small trace of gelatin 



^ • e or none at all have been extensively used for 



Q ultraviolet photography, but these plates were 

 6 ^O a . O ' O handmade and quite expensive. Another method 



O • ^ ■ ^ O ^^^^ used to increase the sensitivity in the ultra- 



o V ' 0*rs violet consists in dissolving the gelatin of an 



. • O e *' ordinary plate. A method which has been 

 * - " ^ . • • - successfully applied by Lyman and others consists 



. ♦ ci, _ '; in bathing the plate with some fluorescent mate- 



iO' O ' *• rial, such as mineral oil, which transforms the 



•^ short wavelengths into longer ones capable of 



penetrating the gelatin and thus exposing the 

 plate. Burroughs, of the Eastman Laboratories, 

 found in his researches on the fluorescent prop- 

 erties of a large number of organic substances 

 J .***"■ **• that the ethyl carboxylic ester of dihydro- 



credladine was particularly satisfactory as an 



ultraviolet-sensitizing medium for ordinary plates. 



This substance is insoluble in water but can be 



dissolved in certain organic substances. When a 



_^ ■ Qv- *> plate is bathed in a solution of it and allowed to 



, ■ * > * ' dry, the surface of the plate is covered with a 



* ' , ,, microcrystalline deposit which fluoresces strongly 



> ^_ ' r\- • * under ultraviolet light. The fluorescent materials 



• ^r^ after exposure of the plate can be removed by 



washing the plate with acetone before develop- 



,. ' ■ * ° O ment or, if desired, by brushing the plate with a 



'"' .9, o "•- tJP ^ camel's-hair brush as soon as it is in the developer. 



* ' rs ' ' • -^o - "^^^ crystals loosen and are thus brushed off. 



> ""■ ' * • •?*.'*•*'" " .-. The sensitized plates are supplied ready for use, 



o . . ,* ..••.■«, or the material may be secured with which to 



O sensitize plates. 



.6 O o , • <? ' O Plates, plateholders, and camera of the ultra- 



• °AN ' ,. • . ^ O violet microscope must be clean and free of dust, 



',.° .. Q^Q , otherwise speckled plates arising from dust or 



■ •' . ' o ' paper fibers on the plate surface will be sure to 



I • ' ' result. Some plates are not carefully wrapped, 



"^ . and during manufacture, shipment, and handling 



O the sensitized surfaces seem to collect a great 



Fig. 26. — Rubber latex particles deal of dust particles which are difficult to remove 



clearly resolved by means of ultra- completely. For ordinary photographic work the 



violet fight. Optical sections spaced ^^^^^ j-^^^ particles of glass, etc., which lodge on 



I4 micron apart. (Magnification ,,,',. 1 , ,1 • 1 jv. 



1800 X ) plate surfaces may be brushed or wiped on 



reasonablj^ well. \Mien the plate is used for ultra- 

 violet microscopy, every little particle completely blocks out the ultraviolet light and, of 

 course, shows up as an unexposed spot on the developed negative. Dust particles 



