THE ACTION OF LIGHT .569 



as Weigert points out (1911, p. 106), we obtain the astonishing efficiency of 

 98 per cent. But it must be remembered that the experiments were not made 

 on the same leaf and also, according to Blackman, the position and distribution of 

 the chlorophyll should be taken into account, which make the percentage of 

 incident light absorbed by it 10 per cent, instead of 4 '2 per cent, and the efficiency 

 is reduced to 41 per cent. In any case, the maximum efficiency is a high one and 

 is, of course, only to be obtained under exceptional conditions. The usual one 

 appears to be about 20 per cent. 



THE ACTION OF ULTRA-VIOLET LIGHT 



The greater number of the constituents of living cells are colourless, that is, 

 they do not absorb rays of the wave length of visible light. Many of them, however, 

 absorb ultra-violet light so that it is not surprising to find that radiations of this 

 kind have a very powerful effect on living cells as a rule. 



The use of the absorbing power for ultra-violet of some constituents of living cells for the 

 purpose of photographing them has been referred to above (page 9), as also the use of the 

 fluorescence excited in them by ultra-violet light absorbed. 



A series of important researches on ultra-violet light is at present being carried 

 on by Victor Henri with several coadjutors, the results of some of which have 

 been published (see Mme. V. Henri, Victor Henri, J. Larguier des Bancels, and 

 R. Wurmser, 1912). 



The first part of this work consisted in the determination of the wave lengths 

 of the light emitted by various sources of ultra-violet light and of the absorption of 

 screens. For the purpose of investigation it is clearly useful to have screens 

 which will cut off ultra-violet and transmit visible light and others which will cut 

 off the visible rays and transmit the ultra-violet rays. The most useful of the 

 former was found to be " euphos " glass, which, in a thickness of 0'75 mm., cuts off 

 very little of the visible spectrum, but only allows a very small amount of ultra- 

 violet to pass. For the latter purpose, a colloidal solution of silver, prepared by 

 the electrolytic method, is valuable. In a thickness of 20 mm., this allows no 

 visible rays to pass, but is fairly transparent to ultra-violet, even waves as short as 

 219 fjifj, are slightly transmitted. In 10 mm. thickness, about 5 per cent, of the 

 red, yellow, and green rays pass, but as much as 30 per cent, of the ultra-violet, in 

 its middle region. Lehmann's (1910) modification of Wood's filter is much used. 

 This consists of a double cell of Jena " uviol " glass, of 2 mm. thickness, that is, 

 6 mrn. of the glass in all. The depth of each chamber is 5 mm. One is filled with 

 saturated copper sulphate, the other with a solution of nitroso-dimethylaniline in a 

 strength of one part in 12,000. The filament of an incandescent lamp is just 

 visible through it, while it transmits a considerable amount of ultra-violet. 



The absorption of egg- and serum-albumin was next studied. In the former 

 the absorption is feeble for rays longer than 300 /x/x, increases to a maximum 

 absorption band at 280 /x/x, has a minimum again at 250 /x/x, and then rises again; 

 so that, for the extreme ultra-violet, the extinction coefficient exceeds 1,000. It 

 may be noted that the mercury arc in a quartz tube sends out rays, of moderate 

 intensity, as short as 220 /tx/x, for which the absorption coefficient of albumin is 

 over 1,000, and intense rays as far as 238 /x/x, for which the absorption coefficient 

 is nearly 200, so that it is not surprising that protoplasm is very sensitive to the 

 extreme ultra-violet of this lamp, as we shall see. 



It has long been known that various small animals, such as those tiny Crustacea 

 found in fresh water, flee from places illuminated by ultra-violet light, and Mme. 

 V. Henri and Victor Henri (1912, pp. 12-21) have found that Cyclops is very 

 useful for experiment. If it be illuminated for two to five minutes with the 

 quartz mercury arc, it first shows great agitation, then becomes immobile. In 

 this state, if kept in the dark, it remains for some hours motionless, but very 

 sensitive to renewed illumination, to which it responds by a vigorous movement. 

 It is thus easy to make exact measurements with it. After a day in water, it has 

 become normal again. 



A certain minimal duration of illumination is necessary for a reaction to 



