SCIENCE OF SEEING

Wairarapa Times-Age, 18 July 1939, Page 9

SCIENCE OF SEEING

INTERESTING FACTS ABOUT THE EYES. RESULTS OF LENGTHY RESEARCH. The unassisted eye has a field of view extending over practically a hemisphere and gives comparatively rapid changes in focussing, allowing moving objects and changing distances to be adjusted almost simultaneously. The eye responds best to the wavelengths actually most abundant in daylight and moonlight, thus demonstrating the strain entailed when driving on a dark night and solely dependant on artificial illumination.

Most people are aware of the comparison of the eye to a photographic camera, but that comparison does not go very far to explain the sensation of vision.

As the result of years of research the detailed structure of the retina—the sensitive coat at the back of the eye—and its connection with the optic nerve and thus to the brain, baffled histologists in interpreting how light impressions were conveyed to the brain and also how a mental picture was actually formed. The light sensitive elements are of two kinds, the rods and the cones, sc called because of their shape. The rods are believed to be the only ones used in night vision and do not distinguish colours with any certainty.

The cones are more abundant in the centre of the field of view where vision is most acute, and it seems to be fairly conclusive that each cone has its own individual nervous communication with the brain.

When light falls on the retina it stimulates the nerves in the same way as a contact with the skin by a foreign substance causes the sensation of touch. In the Visual nervous mechanism the sensations, of course, are much more complex as the light coming from the object viewed with stimulate in three different ways, in the sense of light, colour and form, all being recorded in the brain simultaneously. The light sense is the faculty which permits us to perceive light not only as such, but also its graduations of intensity. By the shading and irregularity of the. reflecting surfaces of an object we are able to interpret its form. The colour sense varies with individuals, some being more acute than others, and also varies with the amount and type of illumination, as different wave-lengths can have the effect of altering a particular colour considerably. If the retina was exposed to light of anv great intensity it is doubtful if it would continue to be useful for the whole term of our natural lives, and sc to safeguard this delicate structure an insulating fluid is present in the retina. As the intensity of the light increases. the pupils naturally become smaller to allow the entrance of less light, and this insulating fluid, the visual purple, is secreted chiefly between the rods, thus minimising the exhausting effects of the light. When motoring against bright headlights the glare is in marked contrast to the dark road and this insulating fluid is stimulated to give protection and better vision. When the approaching car passes, this fluid has to flow back to the layer from which it is secreted before objects can be seen clearly. The fact that some persons sec relatively worse at night than others may be due to imperfect action in the secretion of this visual purple. In some cases of malnutrition in the Great War the visual purple was reduced in quality because of the absence of certain necessary vitamins. Together with the sense of light, form and colour, persons using both eyes should have what is termed stereoscopic vision. This sense of stereoscopic vision is caused by the fusion of two slightly dissimilar mental pictures, resulting in one picture in which objects stand out in relief. This enables us to judge their distance and size more accurately, and gives us a perception of depth and distance of any object viewed. If a small object is placed on a table and an attempt made to pick it up with one eye closed, it will be found that the task is much more difficult than when both eyes aie used. This stereoscopic sense is one of the wonders of our visual apparatus, and its efficiency varies with the individual. governed to a large extent by the intricate muscular system of the eyes and the mental processes which control those muscles through the nervous system.