STUDIES IN SPECIAL SENSE PHYSIOLOGY 379 



which were imposed upon them by imperfect means of investiga- 

 tion. Even with modern apparatus, it is not easy to obtain a pre- 

 cise idea of the elaborate structures contained in the eye as every 

 student knows hence workers unprovided with the simplest 

 microscope knew almost nothing of what is now common know- 

 ledge. Roughly, we may say that all the early theories agree in 

 regarding the " pupil " of the eye and the " image " within it as 

 of primary importance. Again, the flash of light seen on pressing 

 or rapidly moving the eye was held to prove the existence of an 

 inherent or native " fire," also of great significance. Thirdly, the 

 presence of a watery substance within the eyeball had to be 

 accounted for. The problem, as it presented itself to the earliest 

 writers, was to assign their proper shares in the visual act to the 

 " fire," the " image," and the " water." 



One of the earliest of the Greek writers on this subject was 

 Alcmaeon of Cretona (fl. B.C. 500). Our knowledge of his views 

 seems fragmentary ; he thought that seeing is accomplished by 

 rays passing from the ocular " fire " to the object, and that these 

 returning to the eye, altered in some way, are reflected in the 

 diaphanous " water." The " fire " is therefore the active element 

 in vision. The hypothesis hardly appears consistent with itself, 

 because the conception of a visual ray from the " fire " cannot 

 readily be harmonised with a mirror-like action of the " water." 



Empedocles' (circa B.C. 456) theory was more subtle and 

 elaborate, although it is not easy to reconcile different statements 

 attributed to him. 



According to the first doctrine enunciated by Empedocles, 

 like perceives like. All bodies whatever are characterised by 



(1) All are made up of four elements, earth, air, fire, and water. 



(2) All are permeated by minute passages or pores, and all give 

 off emanations which enter the pores. Thus, in perception, emana- 

 tions from the object pass into the pores of the percipient organ. 1 

 But, that this passage may be effected, it is necessary that the 

 emanations and the pores should correspond ; if the former are 

 too large or too small for the latter, no perception can occur. 



Hence with the eye alone can we perceive emanations of colour 



1 Cf. Lucretius, De Her. Nat., bk. ii., 833 



" Noscere ut hinc possis prius omnein efflare colorem 

 Particulas, quain discedaut ad semina ivruin." 



See also bk. iv., 72-86. 



