STIMULATION THE SENSES 113 



a medium which is not material, at least, not matter in the ordinary 

 form. 



Some idea of the kind of vibration in question may be gathered 

 by watching a sea-gull floating on the sea. It will be seen to rise 

 and fall, as a wave passes under it, without permanently altering its 

 position in relation to objects around it. Such a vibration is in 

 one direction only, the vertical, and in the case of light would be 

 called a polarised beam. Ordinary light consists of vibrations in 

 all directions at the various angles with this. 



We have already seen that large quantities of energy are trans- 

 mitted to us from the sun by wave motion of this kind, and that it 

 is only a limited range of wave lengths or rates of vibration that 

 we perceive as light, although the longer wave lengths can be per- 

 ceived by the heat receptors of the skin and the shorter ones have 

 powerful chemical effects. Waves of a greater length than the 

 longest of the solar spectrum can be produced by electric dis- 

 charges, and form the basis of wireless telegraphy. Waves much 

 shorter than the ultra-violet of the spectrum are known as X-rays 

 or Ron tgen -rays', which have remarkable powers of penetrating 

 substances opaque to ordinary light. 



The manner in which the vibrations of wireless telegraphy, 

 electric waves, are produced, reminds us that light is an electrical 

 disturbance, although there are still difficulties to be explained in 

 connection with the relationship between the moving electrons and 

 the transmission in wave form. 



The first question that arises in connection with the perception 

 of light is, what effects capable of being used to stimulate nerve 

 fibres does light produce when it falls upon material objects ? 

 Although, as would be expected, there are certain electrical effects 

 to be detected, the most obvious ones are heat and chemical change. 

 The means of perceiving the former are not nearly delicate enough, 

 and there is every reason to believe that the immediate cause of 

 the stimulation of the endings of the optic nerve is by a so-called 

 photo-chemical reaction. The photographic plate shows us how 

 sensitive such a reaction can be made, although the mechanism in the 

 eye is much more sensitive than the most rapid plate. The fact of 

 a chemical change produced by light is readily seen in the case of 

 " printing out paper." The change does not go so far on the dry 

 plate used in the camera as to be visible, but the fact that an image 

 appears on development by a reducing agent shows that a chemical 

 effect had been brought about. 



The skin of some lower organisms appears to be sensitive to 

 light, but such a general sensibility would only give information 

 of the approach of another object by the shadow cast by it, and it 

 is not until specialised eyes are developed that the perception of 



