354 PHYSIOLOGY CHAP. 



Different opinions have been held at different times in regard 

 to the penetrability of the ultra-violet rays through the eye. 

 The view put forward by Briicke (1845) is now generally accepted, 

 that they are almost completely absorbed by the transparent 

 media of the eye and by the lens in particular. In fact the 

 researches of Chardonnet, which were confirmed by Gayet and by 

 Widmark, showed that patients who had been operated on for 

 cataract perceived the more refrangible part of the spectrum, which 

 is invisible to the normal eye. So that, unlike the ultra-red rays, 

 the ultra-violet are absorbed by the lens, and after its removal 

 they become visible to a certain extent. According to Widmark, 

 the visibility of the most refrangible end of the spectrum varies 

 for the human eye between rays of 395 ^ and of 371 HP wave- 

 length, while in the absence of the lens it extends to the rays of 

 313 P./J.. According, on the contrary, to Mascart and others the 

 extreme limit of visibility extends to the rays of 210 pp.. Beyond 

 this the ether vibrations become so rapid that they no longer 

 excite the receptors of the retina, just as sound-waves that are too 

 short fail to throw the receptors of the cochlea into activity. The 

 Kontgen rays are invisible owing to their excessively short 

 wave-length, but according to Dohrn and others they become 

 visible to a certain extent when very intense. 



It follows that ether waves of moderate length alone are 

 capable of exciting the sensory elements of the retina, and that 

 what we call light and colour depend on the intrinsic property of 

 our receptor apparatus to react to certain vibrations only. We 

 can imagine the existence of eyes, sensitive not like ours to the 

 medium, but solely to the extreme vibrations of the ether. 

 Such an eye would view the world under an aspect very 

 different from that in which we see it ; the thermal or the 

 chemical radiations would be perceptible in the form of colours 

 never seen by us, and which we are unable to imagine, because 

 they are qualitatively different from those which our vision can 

 appreciate. 



All colours found in nature or produced by art can be obtained 

 from the colours of the solar spectrum, although it does not 

 contain the whole of the visible rays, since those corresponding to 

 Fraunhofer's lines are wanting. The mixture of all the spectral 

 colours, in the proportion in which they exist in the spectrum, 

 produces the white light of day. Grey is only white of low 

 intensity ; black is the colour of the objects that do not give off or 

 reflect rays capable of exciting the retina. 



The different sources of artificial light contain different rays 

 in different proportions. Strontium light, for instance, contains 

 chiefly red rays ; sodium light, yellow rays. When these and the 

 other artificial lights are analysed by means of a prism, the 

 spectra obtained are not continuous, but consist of distinct bands, 



