The painter is at liberty to make what use he will of colour so long as he provides a thing of beauty, but he must remember that the appreciation of colour harmony is dependent not only upon the condition of the optic nerves of the observer, but also upon his experience at the time of observation. As to the first consideration little heed need be taken, because rudimentarily the nerve structure is equal or nearly so, in all persons, and while accident at birth may provide in some an advanced condition which in others is only obtained by exercise, yet in respect of colours, experience in complex harmony is gained involuntarily in contact with every-day nature. Hence for the purpose of the painter, all men may be considered alike in regard to the recognition of colour harmony. But individual experience at the time of observation of a painting varies largely,[68] a circumstance which is not of importance in dealing with works of the higher art, but becomes of great significance when considering the lower forms. No organ of the body is so susceptible to fatigue as the eye, and a painting of the kind known as a colour scheme may or may not be pleasing according as the tone is a relief or otherwise to the sight. Sometimes a few seconds are sufficient to fatigue the eye, as for instance when it is directed towards a vivid maroon hanging, but let a landscape with a grey tone be placed on the hanging and considerable pleasure will be involuntarily experienced through the relief to the optic nerves. Remove the picture to a grey wall, and it will instantly lose its charm, except such as it may possess apart from the colour.