VISIBLE AND NEAR-VISIBLE RADIATION 169 



solid, and mixtures in various proportions of ethyl alcohol, index 1.36 

 to 1.37, benzene, 1.49 to 1.51, and carbon disulfide, 1.62 to 1.635, for 

 the liquid, which enables one to obtain any desired wave-length of coinci- 

 dence. In the ultra-violet, quartz is used instead of glass. Particles 

 may be selected of fairly uniform size anywhere from 1 to 3 mm. in 

 diameter. The coarser the particles the thicker the cell which is required. 

 For 2-mm. particles, the thickness would be from 3 to 4 cm., depending 

 upon the spectral purity required. For further details, the reader is 

 referred to a recent paper (45). 



Wherever filter-source combinations are used, producing a spread of 

 intensities over a wave-length range, it must be borne in mind that 

 three things must be taken into consideration in arriving at an inter- 

 pretation of observed effects: (a) The intensity distribution of wave- 

 length from the source. (6) The transmission characteristics of the filter. 

 (c) The absorption characteristics of the material irradiated. The range 

 of wave-lengths actually effective may appear surprisingly different from 

 those which one would off-hand expect from the filter characteristics. 



Consider an example where one is dealing with a photochemical 

 reaction arising from chlorophyll in ether. Thus, in Fig. 14a (64) we 

 have the transmission characteristics of two Christiansen filters, (a) where 

 a cell of 4 cm. thickness is placed in a critical optical system with small 

 restricting apertures (full line), and (b) (dash-dot) where a cell of the same 

 thickness is placed in an optical system composed of a motion-picture 

 projection-lamp filament and a mirror producing a magnification of the 

 source to an area of 4 cm. wide by 8 cm. high. Figure 146 shows the 

 radiation transmitted by these filters from a 100-watt lamp, assuming all 

 the radiation utilized. (To compute the radiant power per centimeter 

 wave-length actually available one would have to multiply by the frac- 

 tion of the entire solid angle subtended by the filter. If a lamp of 

 larger filament area, but the same temperature, 2755° K, were used, this 

 figure should be multiplied by the filament area. To compute total 

 power, one must integrate over the wave-length range, taking wave- 

 length in centimeters.) Figure 14c (70) shows the absorption coefficient 

 (base 10) or specific transmissive index of chlorophyll A in ether. Figure 

 lid shows the energy absorbed by chlorophyll A under these conditions 

 (if such a thickness of cell and concentration is used as to absorb 75 per 

 cent at the red maximum). Curve 1 (d) indicating the radiation 

 absorbed where a precise optical system is used, presents two maxima, 

 one in the region of greatest illumination, and the other occurring in the 

 region of greatest absorption. Curve 2 (d) shows the radiation absorbed 

 where the filter has been used to produce an extended area of illumination. 

 (As before, the figures are for the whole solid angle.) Here the absorp- 

 tion occurring near the region of principal illumination has been shifted 



