T,S() THE SCOPE OF RADIOLOGY. 



plants and animals. The colour of a plant is frenerally due to- 

 the presence of certain pigments. These pigments absorb cer- 

 tain radiations of the incident light and reflect or scatter back 

 the remainder. The scattered light determines the surface 

 colour of the bod}'. The energy of the absorbed radiations is 

 generally utilized by Nature to promote growth. The radiations 

 of the red end of the spectrum are those usually employed to 

 promote metabolism, whereas the action of the radiations of the 

 violet end is katabolic. The green colouring matter in plants — 

 chlorophyll — absorbs the blue rays of light, and converts them 

 into beneficial red rays. The action cif pigments in plants is 

 further demonstrated in the case of seaweeds. At the surface 

 they are green, and the colour gradually changes with the depths 

 at which they are found ; thus they finally de^'elol■) to red. Sun- 

 light, as it passes through the water, is gradually absorbed, the 

 red rays heing absorbed more readily than the blue rays ; hence 

 the latter penetrate to the greater de})ths. At this depth we 

 find only the red seaweeds. The pigment in these plants con- 

 verts the destructive 1)1 ue rays into beneficial orange and red rays. 



Both in plants and animals the pigments protect the under- 

 lying tissues from destructive rays; thus sunljurn. which is caused 

 Ijy the -hort, that is blue, rays is generall}' followed by pigmenta- 

 tion. The continual ex])osure to the action of the sun possil:)lv 

 caused the ft)rmation of melanins, the colouring matter in the 

 skin of the coloured races. This pigmentation forms a protec- 

 tion ; thus the negro or kaffir has a better chance of existence in 

 his birthday suit than a white man, when both are exi)osed to 

 excessive solar radiations. 



It has been found that Avhite cows, fed on buck-hay and 

 exposed to the sun's rays, get a rash, whereas coloured cows do 

 not suffer from the ra;|h on account of the protective pigments. 

 Artificial colouring of a white cow acts in the same manner. 

 Another detrimental effect of blue rays is that they cause pitting 

 in the skin of a smail-pox patient, hence the patient is ])laced 

 where only red rays will reach him ; but it has been found that 

 the blue rays as well as the red rays are beneficial in the cases 

 of certain skin diseases. It may be that the beneficial effect 

 caused bv the blue ravs is the result of pigmentation combined 

 with the formation of antitoxins, hence the good results obtained 

 by " sun-bath cures." When indulging in a sim-bath the head 

 should ])e protected, because the yellow-green rays cause sun- 

 stroke. 



From what has been said above, it follows that it is advisable 

 to wear vellow-green or \\'hite headgear, as these colours absorb 

 or scatter, respectively, the injurious rays. 



Speaking of the effect of actinic ravs in the case of skin 

 diseases brings us to the bactericidal action of these ravs. W'e 

 have already i)ointed (Utt that, in the case of plants, blue ravs 

 destroy life, whereas the red encourage growth. In general the 

 effect of these ravs on bacteria is analogous with that on ])lants. 

 We all know that stagnant water is apt to be the stronghold of 



