VISUAL SENSATIONS 635 



are refracted the least being red, while those which are most refracted 

 are blue. Between these two extremes we have rays of the following 

 colours orange, yellow, green, blue, indigo, which merge one into 

 the other without any perceptible break. The different parts of the 

 visible spectrum, when obtained from the sun, show vertical dark 

 lines, which are known as Fraunhofer's lines. These are due to the fact 

 that certain rays emitted by the glowing centre of the sun are absorbed 

 in passing through the gaseous envelope which surrounds the sun. The 

 lines are distinguished by certain letters and have all been assigned to 

 the existence of known elements in a gaseous form in the solar envelope. 

 Any part of the spectrum is distinguished according to its relation 

 to these lines, since each of them has a constant wave-length. The 

 visible spectrum extends from the line A at the limit of the red, which 

 has a wave-length of 760 millionths of a millimetre, to the line H 

 at the end of the violet with a wave-length of 397. The visible part 

 of the spectrum does not, however, include even the majority of the 

 rays which arrive at the earth from the sun. Beyond the red we get 

 the ultra-red rays, which have a large amount of energy, so 

 that their presence can be easily detected by their warming effect 

 on blackened bodies, such as the blackened bulb of a thermometer 

 or a thermo- junction, held in this part of the spectrum. In the same 

 way, beyond the violet end there is a long extent of rays with high 

 refrangibility and small wave-length. Though not perceptible to the 

 eye, they reveal their existence by the marked influence they exert on 

 salts of silver ; they are therefore often spoken of as the actinic or 

 photographic rays. 



If the investigation of the constituent rays of the solar spectrum 

 were carried out at a considerable altitude above the sea and by means 

 of quartz prisms and lenses, the extent of the invisible spectrum 

 would be found to be largely increased, since the ultra-red and ultra- 

 violet rays are absorbed by the constituents, especially the aqueous 

 vapour, of the atmosphere. Why can we not see these rays as well as 

 those in the middle of the spectrum ? Is it that they are absorbed by 

 the media of the eye, through which the ray of light has to travel 

 before it reaches the retina, or is their invisibility due to an actual 

 insensibility of the retina to rays of high and low wave-lengths ? 

 On testing the absorption of these rays by the transparent media 

 of the eye, we find that the absorption of the ultra-red rays is but 

 slight and that a considerable proportion of these rays must be always 

 arriving at the retina, so that their invisibility must be determined 

 by the fact that they are unable to excite the special sensory elements 

 of the retina. On the other hand, the absorption of the ultra-violet 

 rays by the eye media is practically complete, although these rays on 

 arriving at the retina have the power of evoking sensation. Thus it 



