398 GENERAL PHYSIOLOGY 



brates, such as the earth-worm, eyes are completely wanting, and 

 only the cells of the skin are similarly sensitive. Many 

 unicellular forms possess the power of reacting to photic 

 stimuli, even those that have no organs specially developed 

 for the perception of light, and in the chlorophyllaceous Protista 

 and plants the irritability to light is generally wide-spread. 



On theother hand, there are a host of cell-forms, as, e.^'., the majority 

 of tissue-cells and ciliate Infusoria, which, according to experiments 

 thus far, are not affected in the slightest degree by light-stimuli 

 when the thermal effect of the latter is excluded. But recently 

 an observation has been made, which deserves great attention 

 in considering the question as to the irritability to light of such 

 cells as hitherto have been regarded as insensitive. 



The development of modern electrical technique has revealed 

 methods of producing electric light of very enormous power, which 

 surpasses sunlight in intensity, and which is not sufficiently 

 described by the common word " dazzling." The term "destructive " 

 or " destructively luminous " should be applied to it ; for in the 

 electric works, where labourers are exposed to such light, it has often 

 been observed that the skin of these persons exhibits genuine 

 phenomena of necrosis in the uncovered parts of the body. The cells 

 of the epidermis die, the upper layers of the skin scale off, and the 

 lower layers show signs of intense inflammation and ulcerations 

 like burns. It is not the thermal effects of the light that are 

 shown in these phenomena, but the chemical effects of those rays of 

 the spectrum that have a short wave-length ; this can be determined 

 by inserting media that absorb the heat. Hence there can be no 

 doubt that we have to do here with very strong photic effects upon 

 cells whose living substance is affected only in very slight measure 

 by the intensity of the light-rays that under usual circumstances 

 come to the earth's surface. 



This fact is worth consideration, for it raises the question 

 whether cell-forms whose living substance has been regarded as 

 wholly insensitive to light of the usual grades of intensity do not 

 react to photic stimuli of greater intensities, and, moreover, whether 

 all living substance, just as it reacts to heat, is not also influenced 

 by light, its different varieties responding to different intensities. 

 This jDossibility must certainly be weighed. Yet, so long as 

 conclusive experiments upon this point are wanting — and such can 

 be carried out with little difficulty and in a short time in a large 

 electrical establishment — we must hold to the facts as stated 

 above. 



Absolute darkness can best be considered as the indifferent 

 point, i.e., that point of intensity at which light exerts no stimu- 

 lating effect. Beyond this point with increasing intensity the 

 stimulating effect begins. 



