March 1, 1921 



THE INDIA RUBBER WORLD 



415 



the same room, abundantly supplied with incandescent lamps and 

 reflectors, will have shadows of obscure darkness in all these 

 places. The large portion of green rays in mercury-vapor light 

 results in the practical elimination of obscure shadows, so that 

 -a room illuminated with the proper general intensity by mercur>'- 

 vapor lamps is as free from objectionable shadows as it is in 

 full daylight. 



The writer recently had an opportunity of putting this state- 

 ment to a public test. A meeting, mostly of manufacturers, 

 was being held in a fairly large hall in which there was a well- 

 arranged installation of mercury-vapor lamps. A number of 

 pieces of machinery were in the room for exhibition purposes, 

 besides various tables and other objects. Those present were re- 

 quested to find a place anywhere in the room in which 8-point 

 tj'pe (the size of print on this page) could not be easily read. 

 No such place could be found, though the deepest shadows 

 directly underneath the tables, machines and other objects were 

 diligently tried. There is no doubt about the fact, and it is 

 just what the psychologist would expect as a result of the 

 "Purkinje effect." It is the comparatively high visual power of 

 _grcen light at low intensities that "docs the trick." 



COLOR NOT ESSENTIAL TO VISION FOR PRACTICAL PURPOSES 



Color is not an essential element of vision ; but is simply 

 ornamental. Many people are partially, and a few wholly color- 

 blind ; and yet they hardly appreciate the defect in their vision, 

 and suffer no practical inconvenience. Evolutionists tell us that 

 -color vision is a rather recent acquirement, and that our pre- 

 historic ancestors had no sense of color but saw only differences 

 in light and shade. In industrial lighting, even in those branches 

 in which color forms a part of the art, as in textiles, the color 

 •of the light is of surprisingly little practical importance. Aside 

 from the purely artistic handling of color, which is always done 

 by daylight, and which is entirely distinct from the mechanical 

 manipulation of the colored materials, there is no occasion for 

 1he workman to concern himself with color, except possibly to 

 distinguish one material from another in some cases; and then 

 the apparent colors are of no consequence, but only their dif- 

 ferences. The shifting of the color scale by a colored light, such 

 as that of the mercury-vapor lamp, is therefore of very little 

 practical account in the industries. There are not a few cases 

 in which the exaggeration of difference between certain shades 

 ■or colors is of very material assistance. 



WHAT IS GLARE? 



A familiar defect in a photograph is that known as halation, 

 which results from some part of the field being very much 

 brighter than the rest of it, as a window in an interior. Our 

 eye-camera is subject to the same difficulty; but in vision we 

 call it glare. With the possible exception of the darkness of 

 shadows previously mentioned, glare is the most serious defect 

 in artificial lighting. As miglit be expected, glare has been 

 more talked of and written about than any other problem in the 

 whole subject of illumination. .Mso, quite naturally, there has 

 been much darkening of counsel with words without knowledge, 

 not to mention some pure faking. The importance of the subject 

 demands that we give it careful consideration. The analogy of 

 the photograph will still be helpful. 



Let us first distinguish between glare and daszic : glare is iht 

 result of excessive contrast in brightness between different parts 

 of the i-isual field; dazzling is the effect of light of such high 

 intensity falling upon the retina that it produces an immediate 

 sensation of pain, or acute discomfort. .'\ dazzling light in the 

 field of view will always produce glare, but not all glare is 

 dazzling. The effect of the excessive contrast on the visual 

 picture is the same as it is in the photograph ; the bright spot 

 becomes a patch of white fog, and the rest a shadowy mass 

 with few details distinguishable. Photographically, the bright 

 spot is fogged by overexposure, while the other part is dark 

 and lacking in detail from underexposure. The action in the 



eye is very similar. So far as the result is concerned, the 

 "exposure" in photography depends upon the rapidity of the 

 plate and the size of the stop, as well as upon the time. In the 

 eye the time cannot be varied, but the other two factors can. 

 The iris can contract so as to reduce the amount of light to 

 •about one-fourth that entering at full opening. The regulation 

 of the sensitivity, or rapidity, of the retina is a far more com- 

 plicated process. The eye-camera is furnished with two different 

 sensitive plates, one orthochromatic, i. e., sensitive to all colors, 

 but comparatively "slow," and the other a very "rapid" plate, 

 but incapable of giving color distinctions. The former occupies 

 the center of the retina, and the latter the surrounding surface, 

 the two blending into each other. Referring back to the descrip- 

 tion of the eye, we may call the slow, orthochromatic plate the 

 "cone plate," and the surrounding, rapid plate the "rod plate." 

 In bright light the rod plate becomes insensitive, and ceases to 

 act, leaving the production of vision entirely to the cone plate. 

 In very dim light the cone plate loses its power of action, and 

 vision is produced entirely by the rod plate. The substitution 

 of one plate for the other with the change from bright to dim 

 light takes place slowly, requiring as much as a half-hour in 

 extreme cases. This is called adaptation. The glare produced 

 in going suddenly from a dark to a very light room is due 

 to the excessive overexposure on the rod plate before it can 

 adapt or non-sensitize itself. The effect is increased by the eye 

 receiving the maximum light through the full opening of the iris. 

 The adjustment of the iris, however, takes place comparatively 

 rapidly, within a fraction of a minute. On going suddenly from 

 light to darkness the eye sees nothing until the rod plate can 

 resensitize itself. 



EFFECTS OF GLARE 



Now, what happens when one part of the visual field is very 

 light as compared with the rest? The retina is confronted with 

 the problem of doing two things at once that are of contrary 

 nature; the bright spot is too bright for the rod plate, and the 

 rest of the field, by comparison, too dim for the cone plate. 

 .\s a result, neither is done well and strain or nervous tension 

 is the result — the imperfect vision and ocular discomfort that 

 result from glare. 



There is another condition that undoulitedly serves to ag- 

 gravate the results just described. We have previously stated 

 that the eye naturally seeks out the most prominent point in the 

 field, and focusscs it on the center of the retina. It therefore 

 requires an extra effort to keep any other point in the field 

 focussed for sharp vision. Again, if there are a number of 

 bright points in the field, as in the case of a room lighted with 

 units in each bay, the attempt to focus them all at once leads 

 to conflicting efforts, which must result in muscular strain and 

 nervous discomfort. 



.•\lthough the conditions producing the effects that are col- 

 lectively ascribed to glare are by no means fully understood, the 

 following seem to be fairly established facts : 



Glare is due to excessive contrast in different parts of the 

 visual field. .According to the best information experimentally 

 obtained, the bright spot must be 1,700 times the cube root of 

 the brightness of the general field in order to be glaring. The 

 mathematical fornnila is less important than the simple fact that 

 it is contrast rather than the actual brightness of the offending 

 spot that produces glare. As proof of this the familiar fact may 

 be cited that a bare electric lamp is exceedingly glaring in an 

 otherwise dark room, hut in open daylight is hardly distinguish- 

 able. 



Glare increases with the qxtantily of light entering the eye, as 

 well as with its brightness. 



Glare decreases with the size of the bright spot, and disappears 

 entirely if the spot becomes a visual point. Distant street lamps 

 furnish an example of this fact. 



Glare is said to cease when the bright spot is more than 26 

 degrees from the axis of vision. 



