162 BIOLOGICAL EFFECTS OF RADIATION 



A wide range of filters is available, however, which cut off all shorter 

 wave-lengths and transmit all longer wave-lengths. For convenience, 

 these will be termed short-wave cut-off filters. The transmission curves 



o 



typical of such filters, varying in cut-off from 6500 to 2200 A are indi- 

 cated in Fig. 9. Table 10 (39, cf. pages 458-459) gives specifications of 

 filters, chiefly glass (Corning) and gelatin (Eastman Kodak Company) 

 and approximate short-wave cut-offs from 7200 to 900 A. The cut-off 

 indicated is for practically no transmission. 



A quantitative criterion would be valueless because of the unavoidable 

 variability of the material. Liquid filters of similar characteristics are 

 available. For shorter wave-lengths than 2000 A, the absorption due to 



2P00 4.000 6,000 8.000 10,000 12,000 14,000 A 



Wave -Lenoj+h 

 Fig. 10. — Heat-absorbing filters. Full-line curves show copper chloride in 1.0 cm. 

 cell: 1. One-tenth normal. 2. One-fourth normal. 3. One-half normal. 4. Normal. 

 Dotted curves are glass filters. {Corning.) 



A, Heat resisting, heat absorbing, extra light shade, 2.75 mm. in thickness. B, Ibid., 

 light shade, 2.66 mm. C, Ibid., light medium shade, 2.62 mm. D, Ibid., dark medium 

 shade, 2.97 mm. E, Ibid., dark shade, 2.82 mm. 



air must be taken into consideration. Since many photochemical reac- 

 tions present a long-wave-length threshold, i.e., do not proceed for any 

 longer wave-lengths and do proceed for shorter wave-lengths, short-wave 

 cut-off filters have a wide range of applicability. 



It is often desirable to exclude infra-red radiation, and for this purpose 

 heat-absorbing filters must be used. Glass filters are available whose 

 transmission characteristics are shown in Figure 10 by dotted curves, 

 A to E. For many types of experiments, a liquid filter made of copper 

 chloride or copper sulphate may even better serve the purpose. Charac- 

 teristics of copper chloride (15) are indicated by curves 1 to 4. Diffi- 

 culties are encountered in using copper sulphate because of its low 

 solubility. 



For many types of experiments, a water filter may suffice. In order 

 to enable one to make a selection of suitable thickness. Fig. 11 has been 

 prepared, indicating the percentage transmission and also the energy 

 transmitted per second per centimeter wave-length from a square centi- 



