VISIBLE AND NEAR-VISIBLE RADIATION 201 



Color Vision. — Figure 33, lower section, shows combinations which 

 approximate the trichromatic sensitivity curves of the 1922 standard 

 observer. Such arrangements offer a possibihty for the quantitative 

 determination of the color characteristics of materials. In order to 

 arrive at such quantitative results, the following procedure is necessary: 

 (a) The combined sensitivity, filter plus detector, of the particular 

 arrangement to be used must be determined; (6) on the basis of such 

 actual sensitivity curves, conversion factors must be computed by which 

 the corresponding color data can be found; (c) the sensitivity character- 

 istics of the apparatus must be verified from time to time. 



For calibration and computation of conversion factors, the reader is 

 referred to the Bureau of Standards (42). 



Ultra-violet Phenomena.— The various biological effects of radiation 

 in the ultra-violet have been lumped together and an attempt made to 

 arrive at a universal system of measurements intended to be applicable 

 to all of these various effects. Probably, a group of such effects 

 arises from ionization within the cell. Furthermore, since cells possess 

 certain common characteristics, there is some measure of similarity in 

 the region of effectiveness of these different phenomena. The essential 

 differences may, however, be just the points of paramount importance. 

 In order to bring out these characteristic differences, the relative wave- 

 length distribution curves for a number of these phenomena have been 

 brought together in Fig. 34. The full-line curves in the upper section 

 indicate the percentage of bacteria killed for equal energy dosage: 

 (a) 50 erg/mm.2, (5) iqO erg/mm.^, (c) 250 erg/mm.^, (d) 500 erg/mm.^ 

 for staphylococcus, observed by Gates (32). Dotted curve, e, Fig. 34 

 shows the relative erythemal effectiveness as compiled from a group of 

 observers by Coblentz and associates (24, 25). Dash-dot curves 1, 2, 

 and 3 are equal-energy lethal-effectiveness curves for the alga Chlorella 

 vulgaris, obtained by Meier (52). While these follow fairly closely the 

 bactericidal curves, the dosages corresponding are greater by an order 

 of magnitude or more. Exact comparison is impossible because the 

 percentage kilhng could not be determined in the densitometer methods 

 applied to the algae. In the center section, curve A gives the absorption 

 coefficient for nonirradiated ergosterol, concentration 80 mg. per liter, 

 and for comparison, Curve B, the first product of irradiation (2800 to 

 3100 A) by Reerink and van Wijk (60). While there is some rough 

 similarity between the absorption curve A, which is probably related to 

 the activation of sterols in the production of vitamin D and the lethal- 

 effectiveness curves, one finds little similarity with the erythemal curve. 

 Though we might be well justified in developing a method for evaluating 

 the lethal effectiveness of different sources, there seems to be little basis 

 for an evaluation of therapeutic effectiveness. It does not follow that 



