VISIBLE AND NEAR-VISIBLE RADIATION 



163 



meter area of a tungsten filament lamp, at temperature 2755° K. This 

 is roughly equal to the total energy emitted by a 100-watt lamp if trans- 

 mitted through the indicated thicknesses of water, since the area of the 

 filament is about 1 cm.- 



Typical band-pass filters, which transmit a more or less limited wave- 

 length range are shown in Fig. 12. Since in practice, one often wishes 

 to use a combination of selective emitter with "monochromatic filter," 

 Table 11, a, b, and c, has been prepared, showing some of the possi- 



,9100 rr 



Wave Length 

 10.000 12.000 



14,000 16,000 



E.000 4,000 



6.000 



8,000 



14.000 



16,000 18,000 20,000 A 



10,000 I2p00 



Wave Length 



Fig. 11. — Upper — transmission of water of cell thicknesses: 1. 0.1 cm. 2. 0.5 cm. 

 3. 1.0 cm. 4. 2.0 cm. 5. 5.0 cm. 6. 10.0 cm. 7. 20.0 cm. 8. 100.0 cm. Lower — 

 transmitted radiation in ergs sec. /cm. ^ from tungsten lamp at approximately 2755°K., 

 for corresponding cell thicknesses. (0. direct.) 



bilities of such combinations. With a 250-volt uviarc, operated on 143.5 

 volts and 4.5 amp. at 25 cm., one obtains from a 2-cm. midsection the 

 irradiation values indicated (50). By means of this table, it is possible to 

 compute for a given distance and length of arc, intensities available for 

 such a combination. Since the radiance is fairly uniform over the length 

 of the tube, one may simply multiply by the relative length exposed. 

 Other wave-lengths of monochromatic radiation may conveniently 

 be obtained from helium and hydrogen. Since, however, such sources 

 are not subject to any degree of standardization, it is possible to give 

 only order-of-magnitude ideas as to intensities. At different operating 

 conditions of voltage and current, line intensities vary in both absolute 

 and relative magnitude. Intensities indicated must not be used as the 



