If equal numbers of the eggs of the silk-worm be preserved in a dark room, and exposed to common daylight, a much larger portion of the larva are hatched from the latter than the former.
A great variety of facts might be given in proof, that light exercises an important influence over the processes of development in animal life.
“Those who live in mines, or dark caves,” says Dr. Andrew Combe, “and who are rarely exposed to the light of day, present a pale, relaxed, sallowness of skin, which contrasts with the ruddy freshness of country people and others living much in the open air.”
Every one knows how much paler and more delicate those are who remain mostly, or entirely, within doors. The inhabitants of villages, towns, and cities, may often be known by the lighter color and delicacy of skin, which arises from their occupation being more in the shade than that of the country people.
The extreme paleness and depression of the poor population resident in the dark lanes of crowded cities, medical observers have often noticed, as a striking exemplification of the effects of a want of light.
Light is also well known to have a great influence in preventing deformity.
Those who are confined much within doors, or that are brought up in cellars, mines, and the dark places of cities, where the sun can never come, have much oftener deformity of body than such as have free access to the light.
Among those nations that wear but little or no clothing, thus leaving the system more to the influence of light, it is well known that there is a remarkable freedom from this form of disease.
Rickets is much more common in the dark, dirty parts of cities, than elsewhere.
There is, however, of course, in all these instances, other agencies concerned, than mere light. The effects are more or less modified by a variety of causes, as, for instance, the want of a due circulation of pure air in dark and confined places produces debility of the body, which always tends to deformity and disease. But it is demonstrably true, that the want of light is one of the most prominent among the many causes of bodily deformity.