VISIBLE AND NEAR-VISIBLE RADIATION 



159 



upon and emergent from the sample. A further precaution is necessary 

 in that if appreciable diminution of intensity takes place throughout the 

 sample, different portions of the sample are then subjected to different 

 intensities of radiation and consequently to different resulting effects, 

 which in most cases depend upon the (time X intensity) dosage for a 

 given wave-length. 



The determination of the absolute sensitivity of a selective detector 

 as a function of wave-length is important in view of the great sensitivity 

 available in such commercial detectors, photocells, for example. Pro- 

 vided with a source of monochromatic radiation of any desired wave- 

 length, the procedure is simple. The intensity of the radiation for each 

 wave-length is determined as has been indicated in this section, and the 

 response of the detector measured. If such a detector, however, presents 

 an extended area, the sensitivity should be determined for each portion 

 of the area, since marked differences may exist for different parts. 



FILTERS AND REFLECTORS 



FILTERS 



In a large range of biological experiments, the objections indicated 

 above, due to either the size of the organism or the intensity require- 



2,000 

 Fig. 9.- 



3,000 



4.000 



5.000 



J^OOO 



9.000 



10,000 A 



6.000 7,000 



Wove Length 

 -Short-wave cut-off filters — glass. (Corning.) See Table 10 for identification. 



ments, may be such as to prohibit the use of the monochromator tech- 

 nique. One is then driven to the use of extended sources together with 

 filters. Long discharge tubes or a multiplicity of incandescent lamps 

 may serve to provide such a source. For this purpose, it will be desirable 

 to examine the types of filters available. Very few satisfactory filters 

 can be secured which transmit only a restricted region of spectrum. 



