VISUAL SENSATIONS 569 



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 ex- 

 tends 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 has been observed that after extraction 

 of the lens for cataract the visibility of the spectrum, which in the normal 

 eye only extends to the line H with a wave length of 397, is increased on the 

 violet side so that the spectrum may be seen as far as a point corresponding 

 to a wave length of 313. It is evident from this that in the absorption of the 

 ultra-violet rays by the eye the lens takes a preponderating part. 



LUMINOSITY OF DIFFERENT PARTS OF THE SPECTRUM 

 The physiological nature of our conception of light is shown by the fact 

 that the spectrum differs in its luminosity, i.e. in its total stimulating effect 



