CHAP, vii KETINAL EXCITATION 353 



the solar radiations is not confined exactly to the three different 

 parts of the spectrum. The photic rays of the less refrangible 

 portion, as red, orange, yellow, are also thermal ; the visible rays 

 of the more refrangible portion, as green, blue, indigo, violet, are 

 actinic in proportion to their refrangibility. From this fact we 

 conclude that all the radiations that make up the spectrum as a 

 whole, both visible and invisible, are of the same nature that is, 

 they consist of ether vibrations which differ only by their differing 

 refrangibility or wave-length. 



The visible part of the spectrum, which under ordinary condi- 

 tions is limited by the extremes of red and violet (line A to line 

 H in Fraunhofer's spectrum), may under special experimental con- 

 ditions extend for a certain distance into the ultra-violet region. 

 Helmholtz found that when the ultra-violet part of the spectrum 

 was made to pass through the slit of a diaphragm, and then to 

 fall upon a second prism so as to exclude all the extraneous 

 elements by a new refraction, the eye appreciates a portion 

 of the ultra-violet region, from line H to line R, Fraunhofer, 

 as a lavender-hued sensation. 



The portion of the spectrum which we perceive as coloured 

 consists of rays, the wave-length of which decreases gradually 

 from 760 ^ (line A) to 397 /*/* (line H) ; the ultra-violet portion 

 of the spectrum, which is faintly visible as lavender when all the 

 rest of the spectrum is cut out, lies between 397 and 320 /z/x 

 (line R). The whole of the visible portion of the spectrum thus 

 occupies a less interval than a complete octave ; the dimly visible 

 part occupies an interval of about a minor sixth. We may there- 

 fore conclude that the visibility range of the ether vibrations is 

 much smaller than the audibility range of the vibrations of 

 ponderable bodies. 



On what does the invisibility of the radiations of the extreme 

 regions of the spectrum depend ? Obviously there are two possi- 

 bilities : either the rays may be absorbed by the different trans- 

 parent media of the eye and do not therefore act on the retina 

 or the ultra-red rays are too slow, and the ultra-violet rays too 

 rapid, to excite its sensitive elements. 



Special experiments have been made to measure the absorp- 

 tion of the thermal rays, when they pass through the dioptric 

 mechanism of the eye. The observations made by Aschkinass on" 

 the human eye showed that only rays with a wave-length of 

 872 /z//, are absorbed to an amount of 10 per cent, so that the 

 greater part of the ultra-red rays easily pass through the trans- 

 parent media of the eye without undergoing appreciable absorp- 

 tion. To explain their invisibility we must therefore assume that 

 their vibrations are too slow to excite the receptors of the retina, 

 just as vibrations of unduly prolonged sound fail to excite the 

 receptors in the internal ear. 



VOL. iv 2 A 



