571 



1. The effect is less in the green Hydra than in the colourless one, presumably 

 due to the oxygen afforded by chlorophyll. 



2. Oxyhsemoglobin is reduced. 



3. If alizarin blue is injected into the veins of a rabbit, the brain is blue, but, 

 if acted on by ultra-violet light, the dye is reduced to its colourless derivative. 



An interesting point is that, if rays of equal energy (as measured by the 

 thermopile) are taken, one of 440 /X/M, the other of 280 /*//,, Hertel found that 

 rotifers are killed by the short waves in fifteen seconds, by the long waves only 

 after four or five hours. This fact shows strikingly that it is not a question 

 merely of energy, but of the actual wave length, no doubt because the short waves 

 are absorbed by the protoplasm. 



We have seen how large a number of photo-chemical reactions are brought 

 about by ultra-violet light, so that we may expect to find it active also on 

 protoplasmic systems. But the nature of these reactions in the latter case is still 

 unexplained. One of the great difficulties in the therapeutic application of ultra- 

 violet rays to stop the growth of malignant cells is the very rapid absorption by 

 the superficial cells, so that the active rays do not penetrate more than a short 

 distance. Considerable success has attended the treatment of superficial skin 

 growth, such as lupus, by ultra-violet light. Since haemoglobin has so active an 

 absorption for ultra-violet (see especially the data of Victor Henri, etc., 1912), 

 Finsen (1901, p. 70) uses a method of compressing the blood out of the area of 

 tissue to be acted on by light. 



The great effect of ultra-violet light on the skin is familiar to every one in the 

 inflammation (erythema solare) called sunburn, which results in a brown coloration. . 

 It may be pointed out that this is not an effect of heat, in fact it is more liable to 

 occur in cold surroundings, probably owing in part to the fact that the heat of the 

 sun's rays is not noticed and no means taken to protect the skin from their action. 

 It is, no doubt, due to the products of some photo-chemical reaction, acting on the 

 arterioles, and it would be interesting to know whether, like the effect of oil of 

 mustard, it is an axone reflex in sensory nerve fibres. 



The Eye-Media. Of course, if the eye is exposed to ultra-violet light, severe 

 conjunctivitis is caused. Workers in this light have to wear spectacles impermeable 

 to the short waves and frequently also to protect all parts of the skin exposed. 

 This is especially so in electric welding, since the arc spectrum of iron is very rich 

 in ultra-violet lines. E. K. Martin (1912) finds that the cornea absorbs all rays 

 shorter than 295 pp. The lens is thus protected from the most active rays, 

 although it is capable of absorbing rays between 300 and 400 fj.fi, which might affect 

 it and cause opacity (cataract), unless kept back by a screen outside. It was found, 

 however, that while the mercury arc caused conjunctivitis, no change in the lens 

 could be detected. 



Hallwachs' Effect. There is one further effect of ultra-violet light of theoretic 

 interest in connection with the nature of photo-chemical change. Hallwachs 

 (1888) noticed that a negatively charged insulated metal, such as a gold leaf electro- 

 meter, loses its charge when illuminated with ultra-violet light. This appears to 

 be a case analogous to those in which light energy is stored, since the light energy 

 is changed into the kinetic energy of moving electrons, shot from the metal. 



PHOTO-DYNAMIC SENSITISATION 



Although ultra-violet light has so much more action on protoplasm than visible 

 light has, it has been found by several observers that light which has no action by 

 itself on infusoria, bacteria, or blood produces the effect of ultra-violet light when 

 certain dyestuffs are present. Although the dyes used were, for the most part, 

 fluorescent, this does not seem to be an essential factor. For a complete account 

 of the work, the reader is referred to the monograph by Tappeiner and Jodlbauer 

 (1907). The general nature of the phenomenon will be clear from the few remarks 

 following. Hertel (1905) exposed certain bacteria to light of 448/A/tx. This had 

 no effect on them, either with or without the presence of eosin, 1 part in 1,200. 

 Eosin has no absorption band in this position. Ultra-violet light of 280 fj.fi. killed 



