414 



THE INDIA RUBBER WORLD 



March 1, 1921 



while the picture is being taKen, only the eye turns in all direc- 

 tions instead of merely swinging around. 



WHAT HAPPENS WHEN WE SEE 



The formation of the picture on tlic retina is a simple matter 

 of the mechanics of light ; but the process of seeing this picture, 

 that is, of realizing it in consciousness, is intricate beyond com- 

 prehension. Consider that there are two pictures taken from 

 slightly ditTerent positions and consequently diiTerent in perspec- 

 tive ; that the pictures are bottom side up and reversed right and 

 left; that they are of microscopic fineness; that they have all 

 variations of color as well as of hght and shade; that they are 

 constantly changing with instantaneous swiftness, and that ob- 

 jects in them are frequently in motion: all of this complex, the • 

 visual apparatus in the brain transforms into the single, congruous 

 impression of objects having their actual relative sizes, shapes, 

 motions and positions in space. Furthermore, this transforma- 

 tion does not take place where the picture is produced, but the 

 image is first converted into nerve currents — whatever they may 

 be — and transmitted to the brain through cables of nerve-fibers. 

 If we follow the analogy of photography to the whole visual 

 apparatus, we must imagine a tiny camera an inch long, with the 

 sensitive plate connected by a telegraphic cable directly to a 

 moving picture projector, which gives a life-size picture in colors 

 of whatever the camera "shoots." Science has some distance 

 to go yet before it can equal this "stunt" ! 



But to return to the retinal image as the determining factor 

 in vision. We have seen that mechanical defects in the lens of 

 the eye can be remedied by the use of glasses; but there is one 

 imperfection in the lens of the eye which is inherent and impos- 

 sible of correction by artificial means. The nature of this defect 

 is easy enough to understand. If you look at an object through 

 a simple magnifying glass you will notice a play of rainbow 

 colors about the sharp edges and lines of the object. This 

 results from the fact that a simple convex lens cannot bring the 

 different colors to a focus on the same surface; if the yellow is 

 in focus, the red focus will be back of the surface, and the blue 

 focus in front. This is called chromatic abcrralion. By combin- 

 ing lenses of different kinds of glass it is possible to overcome 

 this difficulty almost entirely. Such a compound lens is said to 

 be achromatic. The lens of the eye is tiot achromatic. The retinal 

 image is therefore subject to chromatic aberration. A perfectly 

 sharp image is formed only when objects are seen by light of 

 one color, or monochromatic light. The more nearly light ap- 

 proaches this quality the sharper the retinal image of objects 

 seen by it. 



SHARPNESS OF VISION DEPENDS UPON COLOR OF LIGHT 



Before the invention of the mercury-vapor lamp the chromatic 

 aberration of the eye had no practical application to the use of 

 light for general illumination. All other light-sources, as we 

 have before mentioned, have full, continuous spectra, i. e., contain 

 all the colors, and the only way to obtain monochromatic light 

 from them is to absorb all the colors except the one desired, 

 which was far too wasteful a process to be seriously considered. 

 In fact, the great object sought in the improvement of artificial 

 light was to get it as nearly white as possible, i. e., of the same 

 color as full daylight. 



When the mercury vapor lamp was first offered to the public 

 the distinct color of its light was generally considered a fatal 

 objection to its use for any purpose except photography, in which 

 its usefulness was at once recognized. It was some years before 

 the fact that the light, on account of its nearly monochromatic 

 character, produced a sharpness of vision quite unobtainable 

 with other artificial light, or even with sunlight. The very 

 manifest advantage of this increase in visual acuity in the case 

 of industrial lighting gradually overcame the objection arising 

 from its unfamiliar, sometimes startling, color effects, and finally 

 placed it in a class by itself as a light to work by. 



The retina resembles the photographic plate in being sensitive 



to different degrees for the different colors, but differs from 

 the plate in the order of its sensibility. The rays which produce 

 the greatest effect upon the visual organs are in the middle of 

 the spectrum — the yellow and yellow-green, while the rays that 

 have the greatest photographic effect are in the extreme blue 

 end and the invisible rays of still shorter wave-length, called 

 the ultra-violet, or actinic rays. The relative brightness of the 

 different parts of the spectrum is indicated in the curves that 

 are shown in Fig. 3. Mercury-vapor light is peculiar in having the 

 largest part of its rays in the most luminous part of the visible 

 spectrum. The different lines of the mercury-vapor spectrum 

 are shown by the heavy lines in the curve. This accounts for the 

 high mechanical efficiency of the mercury-vapor lamp. 



It may be well to explain the above curve by reference to 

 the mechanics of wave motion, with which we started. It was 

 stated that the mental sensation of brightness depends upon the 

 energy of the light-waves. This is true with reference to any 

 particular wave-length (color), but different wave-lengths do not 

 produce the same effects of brightness with the same amounts of 

 energy. ,Red and violet waves having the same amounts of 



Fit;. 3. Curves Showing the Rel.\tive Bright.vess of Different 

 Colors for Equal Amount of Energy 



energy, or physical intensity, produce a very feeble visual effect, 

 or brightness, compared to yellow and green waves having the 

 same energy. The relative degree of brightness of the different 

 colors for equal amounts of energy are what the curve represents. 



APPEARANCE OF COLORS DOES NOT FOLLOW PHYSICAL 

 VARIATION IN LIGHT 



The psychological sensations of color do not closely follow 

 the physical variations in the light-waves. The quality of color 

 changes with its brightness, all colors fading into a bluish gray 

 at very low intensities. If you look at a sample card of dif- 

 ferent colored fabrics arranged in spectral order — red, orange, 

 yellow, green, blue and violet— under a fairly high illumination 

 by sun light, they all show their characteristic color values. If, 

 now, the intensity of the illumination be gradually reduced, the 

 colors will presently begin to change their tone, as well as their 

 brightness ; the lightest part of the color-band will move from the 

 yellow to the green, while the red and orange will become darker, 

 and the blue and violet lighter, i. e., fainter. As the intensity 

 decreases further the colors on each side of the green become 

 less distinct, until they become quite invisible, leaving only the 

 green, which finally gives way to the neutral gray, the color 

 of all things, as well as cats, in the dark. Tliis change of color 

 with change of illumination is called the Purkinje effect, from 

 the name (pronounced Poor-keen'-ye) of the Austrian who first 

 observed it. 



This peculiar psychological phenomenon enters into the general 

 problem of industrial lighting in a very practical way. The im- 

 portant part is this: green light suffices to produce distinct vision 

 at lower intensities than light of any other color. One of the 

 greatest — probably the greatest — defects in artificial lighting as 

 compared to daylight is the darkness of the shadows. Even in a 

 large room with only side windows, it is comparatively easy to 

 see in the darkest shadows, as under benches, tables, etc. ; while 



