Page:Popular Science Monthly Volume 71.djvu/310

304 to the most highly civilized races, till the last few centuries. With the invention of writing and then with the invention of the printing-press a new element was introduced, and one evidently not provided for by the process of evolution. The human eye which had been evolved for distant vision is being forced to perform a new part, one for which it had not been evolved, and for which it is poorly adapted. The difficulty is being daily augmented. The invention of printing presses has been followed by an increasing number of books, magazines and daily papers. The rural population has given place to the urban. The long days of manual labor have given way to the eight-hour system with abundant time for reading. Labor-saving devices of all sorts have added to our sedentary habits. All things seem to be conspiring to make us use our eyes more and more for the very thing for which they are the most poorly adapted. It requires no prophet to foresee that such a perversion in the use of an organ will surely result in a great sacrifice of energy, if not of health and of general efficiency.

The eye has thus far been spoken of as though it consisted merely of delicate muscles, when in reality these are not the most significant part of it. In thinking of the eye we should never disregard the eye-muscles, but primarily the eye is a live camera consisting of a lens, dark box and sensitive plate. The retina in the back part of the eyeball is the sensitive plate and is the most vital part of the eye. It is effected by every ray of light falling upon it. Fortunately it responds to a weak light and still is not injured by a moderately strong one. In speaking of the quantity of light it is well to have a standard. For this purpose the most convenient standard is the amount of light cast by a standard candle upon any point in the horizontal direction one foot from the candle. A light of twice this intensity is spoken of as a two-candle power, a light ten times the first is of course a ten-candle power. The light cast by a candle upon a printed page at a distance of one foot is sufficient for legibility at the normal reading distance. If the light is less than this the retina is not adequately stimulated and the reading is accomplished only after a strain more or less intense. If the light falling upon the page exceeds ten-candle power the stimulation of the retina is so great that it is displeasing to some people and is condemned by our best authorities as injurious to the retina. All are agreed that less than a single candle-power is injurious for reading, and during the present state of our knowledge it is at least safe to avoid an illumination of more than ten-candle power.

The iris may be blue, brown or gray and is that which determines the color of our eyes. It is an adjustable shutter which reflexly regulates the amount of light which enters the eye. In the presence of a