570 PRINCIPLES OF GENERAL PHYSIOLOGY 



take place. This duration is less, the greater the proportion of ultra-violet in 

 the light. Thus, through a quartz screen, the time is about two seconds ; through 

 10 mm. of colloidal silver it is only increased to twenty-five seconds, although the 

 ultra-violet is much diminished by the screen ; through " euphos " glass, no reaction 

 was produced in 200 seconds. 



There is also a minimum intensity of illumination below which no reaction is 

 obtained however long the exposure. There is also a certain intensity of illumina- 

 tion at which the necessary time of exposure is the shortest, an intensity greater 

 or less than this requiring a longer time. 



It is interesting to note that there is also a phenomenon of summation of 

 sub-minimal stimuli, repeated at intervals, similar to that which we have seen in 

 spinal reflexes. 



A remarkable fact is that the excitability to ultra-violet rays is independent of 

 temperature, a fact which shows that the exciting cause of the reflex movement 

 is a photo-chemical reaction, whose products, no doubt, excite receptors of some 

 kind in the outer surface of the animal. These receptors then excite nerve endings. 



Fatigue can be produced to ultra-violet light in Cyclops by very short duration 

 of radiation, if the peripheral receptors have been modified by prolonged previous 

 radiation or by cocaine, but not if the anaesthesia is of central origin, by ether, for 

 example. In this latter case, the peripheral organs are left intact. These facts show 

 that the reaction studied is really due to photo-chemical changes at the periphery. 



Since the absorbing power of protoplasm is very great, especially for the 

 shorter wave lengths of ultra-violet, the effect can penetrate for only a short 

 distance. In such small organisms as bacteria, however, it may affect the whole 

 organism, so that the action, which is a lethal one in such cases, obeys the laws 

 of photo-chemical reactions. In larger organisms, we have to take account of 

 the diffusion of the products of photo-chemical change, and their action at places 

 remote from that where they are formed. The Jaws are, therefore, more complex, 

 and the effects last longer than the actual exposure. 



The following table (from Victor Henri, etc., 1912, p. 33) gives the thickness 

 of a layer of protoplasm which reduces ultra-violet light of different wave lengths 

 to one-tenth its value, or absorbs nine-tenths of it. 



Wave Length Thickness of Proto- 



in fj.fi,. plasm in ft.. 



240-5 - - 79 



238-5 

 231-3 

 226-5 

 219-5 

 214-4 



58 

 18 



9 



6 



3-8 



Action on Bacteria, Tissues, etc. An important practical question is that of the 

 lethal action of ultra-violet light on micro-organisms, since it has been used for 

 sterilisation of water. Equally important is its destructive effect on the tissues of 

 higher animals, as used in the therapeutic method of Finsen. 



Hertel (1905) has studied in some detail the effect of the magnesium line of 

 280 ftfj. on bacteria, protozoa, and some toxins, etc. Bacteria were found to be 

 killed in from fifteen to sixty-five seconds. Mme. V. Henri and V. Henri (1912, 

 pp. 29-31) find that there is no particular wave length which is specially lethal, but 

 that the effect increases more and more the shorter the wave length, as far as 

 tested, that is, up to 214 /x/z. Protozoa are killed in about the same time as bacteria. 

 according to Hertel. The body swells, watery drops appear on the surface, and finally 

 disintegration occurs. Rotifers, nematodes, and molluscs are also killed. On the 

 tadpole, the effect is swelling of the epithelium, followed by migration of pigment 

 and nuclear division, together with stasis of blood in the capillaries. On the cells 

 of Elodea, slowing of the protoplasmic movement was noted, which was much less 

 if illuminated by ordinary light at the same time. A longer exposure is required 

 for tissue cells than for bacteria. 



Diphtheria toxin was destroyed in five minutes, but the antitoxin was not 

 destroyed in thirty minutes. Various enzymes were also rendered inactive. 



Hertel regards the effect as due to reduction processes for three reasons : 



