VISIBLE AND NEAR-VISIBLE RADIATION 127 



In forming a general picture of the wave-length range to be dealt 

 with, we may turn to Fig. 2 (11). In the upper section, the transmissive 

 exponent k (cf. i, Table 5), as a function of wave-length, has been indicated 

 by the full-line curve a. Remembering that the ordinates are on a 

 logarithmic scale, one realizes the significance of the rapid and violent 

 changes in absorption characteristics in proceeding through the range 

 of wave-lengths indicated. Water is so transparent in the blue-green 

 that radiation can penetrate through several hundred meters. On the 

 other hand, at a httle less than 0.2 n, even a layer of 0.25 mm. of water 

 becomes practically opaque. The same is true in the relatively near 

 infra-red in the region of 2.0 ix. Contrast these abrupt changes with the 

 more gradual change indicated by the dotted curve h for the region from 

 0.1 to 10 A (reading the abscissas as Angstroms instead of microns). 



Selectivity is also to be found in the radiation emitted by many 

 materials. The mercury arc, for instance, as shown in the middle sec- 

 tion, emits radiation covering only a small fraction of an Angstrom 

 in spread, at each of a multitude of isolated wave-lengths. A solid body, 

 on the other hand, emits energy over a continuous range of wave-lengths 

 from the deep infra-red rising to a maximum and then falling rapidly 

 as one proceeds to shorter wave-lengths. The wave-length at which this 

 maximum occurs depends upon the temperature of the source. 



Selectivity is again to be found in the interaction of biological material 

 with radiation. For illustration, a number of such effects have been 

 graphically indicated in the lower section. Curve a shows the transmis- 

 sion of 0.5 cm. of flesh; curve 6, the response of the eye, in arbitrary units, 

 to equal energy; curve c, the relative phototropic sensitivity of an oat 

 seedling: curve d, the relative absorption of vitamin A to wave-lengths 

 destructive to its biological effectiveness; curve e, the absorption of 

 ergosterol, at least a portion of which may contribute during irradiation 

 to its activation; curve /, the relative erythemal effectiveness. Thus, 

 we see that for each biological phenomenon the contribution of equal 

 energy but different wave-length will depend in cliaracteristic manner 

 upon wave-length. 



Building upon the concept of radiation as the transfer of energy 

 from one point to another by means of an electromagnetic wave motion, 

 our problem is to determine the amount of energy transferred and the 

 wave-lengths associated with this transfer. Our measurements will 

 be based entirely upon the erg, centimeter, and second, all other units 



radiation, which produces activation yielding therapeutic value of vitamin D. /, Relative 

 erythemal effectiveness, zero degree (very light). For extreme erythema, fourth or fifth 

 degree, the relative intensities of the two maxima are reversed. 



Long wave limit of lethal effect indicated by j D. Position of magnesium line in 

 region of great absorption by ergosterol indicated by Mg | . {For hihHographij of this 

 figure see Brackett, 11.) 



