lOG ON COLOUR-BLINDNESS. 



The usual explanation of tho inabiHI;y of certai'n persons 

 to distinguish certain colours is based on the assumption 

 that the human eye is sensitive to throe and only three 

 colours, which are therefore named " primary." Regarding- 

 the actual colours to be considered as primary there has 

 been some dispute. But if this theory is accepted, its ex- 

 pounders explain colour-blindness by the theory that colour- 

 blind persons are wanting in the ability to per'oeivo some 

 one of these primary colours. 



Now there is absolutely no physiological basis for the 

 assertion that the eye is sensitive to three colours, and that 

 all others are compounded of these three. If it bo true, 

 then perception by the eye is an exception to all other 

 methods of receiving external impressions. 'JMio ear is 

 sensitive to vibrations of the auditory nerve produced by the 

 impact of air-waves. The skin is sensitive to heat vibra- 

 tions. It has been pointed out by the author in a, paper 

 published in the Proceedings of this Society some years ago, 

 that in all probability the sense of smell is due to vibrations 

 of the olfactory nerves ; and Professor Haycraft has recently 

 speculated on a similar cause for taste impressions. Now 

 we know that as to the sense of hearing there is a limit to 

 the number of vibrations per second detectable by the ear ; 

 also that some ears are more sensitive to high notes than 

 others, so that a, sound which impresses one person as a, 

 musical note is inaudible to another, whoso ears are inca- 

 pable of perceiving the sound of such rapid vibrations. The 

 skin, in its ability to perceive heat or cold, is also insensitive 

 to extremes of temperature ; so that a scar from a bright- 

 red-hot piece of iron is almost painless, and is not perceived 

 as heat ; nor does an extremely cold object produce a cold 

 sensation. In all probability, too, the inability to smell ga- 

 seous substances of low molecular weight is caused by the 



