ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 123 



independent and yet they are often interdependent in a way which is not 

 always completely understood. This situation can probably be best pre- 

 sented by examples. Assume the practical problem of the sterilization of 

 large volumes of a liquid by ultraviolet radiation. This is to be done by 

 flowing the liquid in a thin film of large area exposed to the total radiation 

 exclusive of the infrared. For practical reasons it is decided to use the 

 mercury-vapor discharge lamp. The amount of ultraviolet energy 

 needed has been established by previous experiments. 



The mercury discharge lamp has a relatively large fraction of the total 

 ultraviolet output in the biologically potent region around 2600 A but 

 the intrinsic brilliance (steradiancy) is quite low compared with other 

 lamps. This particular problem allows a large area into which the 

 required energy can be delivered. Thus what the mercury discharge tube 

 lacks in steradiancy can be made up by extending the emitting area, which 

 is quite easily accomplished, until the total amount of energy received 

 by a unit volume of liquid during exposure meets the experimental 

 requirement. 



The broad band of radiation to be used in this experiment is most 

 easily isolated by a filter system which can usually also be extended in 

 area at will (see p. 142). 



It is quite clear that, in this case, it would have been uneconomical and 

 difficult to have attempted the use of a source which was very bright, 

 i.e., of high steradiancy. Such sources usually attain brilliance by high 

 current densities in small volumes. The total energy output may there- 

 fore be less than a greatly extended source of low steradiancy. Indeed 

 one of the highest rates of total ultraviolet output has been achieved with 

 such a source of low steradiancy. Furthermore, as can be seen in the 

 general references cited, a high current density is often obtained at the 

 expense of simplicity and ease of operation. 



The steradiancy of a source becomes a matter of importance when for 

 any reason it becomes necessary to use an image of the source for irradia- 

 tion. The whole matter of power transmission through image-forming 

 systems has been considered in detail by Loofbourow (1950) and Blout 

 et al. (1950). 



The importance of a careful study of these principles may be indicated 

 by the example which follows. Assume that only the cytoplasm of a cell 

 is to be irradiated and a study made of the effects of such irradiation on 

 the nucleus. Such an experiment will require the formation of a reduced 

 image of the source or a portion thereof within the cytoplasm. This 

 would probably be accomplished by use of a reflecting objective as a con- 

 denser. In these experiments also the use of the total emission of a 

 source will be assumed. The following relation, known as Lagrange's 

 Law (see Hardy and Perrin, 1932, p. 43), has been shown to hold by a 

 number of writers including those cited. 



