41. 



THE INDIA RUBBER WORLD 



March 1, 1921 



Artificial Lio^htin^ in the Rubber Industry — IIT 



The Fundamental Principles of Illumination (Psychophysical) 



By E. Leavenworth Elliott 



WHAT IS PSYCHOPHYS1CS7 



N THE LAST ISSUE we discusscd the nature and action of light 

 as a form of energy. The subject was one of physics and 



mechanics, having to do only with matters external to the 

 human body. We come now to the consideration of what takes 

 place within the body through the action of light. 



The performance of the mind constitutes the subject of 

 psychology, and the performance of the body the subject of 

 physiology. The subject which deals with the relation between 

 the action of the mind and the action of the body is called 

 psychophysics. Seeing is the mental result of light acting upon 

 the visual organs, and hence belongs to the science of psycho- 

 physics. 



The subject of illumination is something like the manufacture 

 of rubber. You have certain substances which you subject to 

 various mechanical processes, all of which are carried out in ac- 

 cordance with the rules established by experience, and then the 

 product of these mechanical manipulations is subjected to a mys- 

 terious process which you call "vulcanization," which changes 

 the whole mass of materials into a new body having very dif- 

 ferent properties from the combination you started with. No 

 matter how good your materials, and how perfectly the me- 

 chanical operations have been performed, unless the vulcanization 

 takes place properly the final result is a failure. What happens 

 during this process? Nobody knows. To be sure, you have 

 names for all that you can find out about it; you talk of 

 "polymerization," and "rearrangement of atoms in the molecule," 

 but what do you knotv about atoms and molecules? So, we 

 know how to manipulate heat, electricity, and various materials 

 to produce light, and we can direct and modify the light to make 

 it serve our purposes of seeing. But it is what takes place after 

 the light enters the eye that produces the results we are after; 

 the process of seeing is the all-important thing, and this is a 

 far greater mystery than vulcanization. However, you know the 

 conditions required for good vulcanizing without knowing what 

 actually takes place, and we can likewise learn much about the 

 conditions for good seeing without understanding fully just how 

 the eye does it. But the better we understand the construction 

 and working of the eye the belter able we shall be to provide the 

 conditions requisite to its efficient operation. 



THE EYE AS AN OPTICAL INSTRUMENT 



The idea generally conveyed when the eye is mentioned in- 

 cludes all the organs of vision, of which the eye proper, or 

 eyeball, is only a part. Let us examine this visual mechanism 

 in detail. Anatomists tell us that it consists of three main organs: 

 the eye-ball, the optic portions of the brain, and the nerve-cable 

 connecting these. 



It is customary to describe the eye (eye-ball) as a small 

 camera, and this comparison is good as far as it goes ; but the 

 eye is much more than a camera — it is a whole photographic 

 laboratory. However, the similarity between the visual and 

 photographic processes as a whole is so remarkably complete that 

 it furnishes one of the best methods of explaining vision. 



To begin with, the eye is a double mechanism, and hence is a 

 stereoscopic camera. This binocular vision (seeing the same 

 thing with two eyes) is of great assistance in enabling us to 

 judge the relative distances of objects, just as the stereoscope 

 brings out distance in the views seen through it. As an optical 

 apparatus, the eye is exactly similar to the camera. The illustra- 

 tion Fig. 1 represents a section through the eye. The familiar 



term eye-ball is exact ; the eye is a ball, or sphere, bulging 

 slightly in the front. This ball fits a socket in the skull, in 

 which it is free to move to a certain extent in any direction. 

 Si.x muscles attached to the outer surface and around the front 

 oi the ball hold it in place and also turn it in its socket ; it is 

 thus like a camera mounted on its tripod with a ball-and-socket 

 joint. The outside casing of the ball is a tough, fibrous substance, 

 called the sclerotic S. On the front this coating is transparent 



and horn-like, and 

 is called the cornea 

 C. The cornea 

 bulges out in the 

 form of a portion 

 of a smaller sphere, 

 and forms part of 

 the compound lens 

 with which the eye 

 is fitted, the other 

 elements of the 

 combination being 

 the crvstalliiic lens 



Conjunettuf 



Sclerotic S 



■ e 



-.-irlery 



Fig. 1. A Diagram of the Eye 



Fig. 2. Image on 



the Retina — 



Full Size 



•Comimied from Ti!e India RrnnER Wori d I rliriMry 1. 1921, pages 329-332. 



L which is a double convex lens of a solid, transparent substance, 

 and the transparent liquid called the aqueous humor A, filling the 

 space between the lens and cornea. Between these two there is also a 

 membrane having a circular opening, called the iris I, which 

 forms the diaphragm or "stop" of the lens. The iris is the central 

 round spot which gives the characteristic color to the eyes, and 

 the circular opening is called the "pupil." 



The aperture in the iris is capable of opening and closing 

 through a range of about four times the minimum, which it does 

 automatically, and for exactly the same purposes as the size of 

 stop is varied in the camera; that is, it opens in dim light to 

 admit more light, and it closes slightly when the lens is focussed 

 on near objects, which increases the sharpness of the image. 

 It is commonly supposed that the dazzling effect of going from 

 a dark room into a very light room is due to the excessive 

 amount of light that enters the eye through the full opening in 

 the iris, and that this dazzling effect disappears when the iris has 

 had time to contract. This is only a part, and the smaller part, 

 of the explanation ; the principal cause of this effect will be ex- 

 plained later. 



The sclerotic, or outer casing, is lined with a thin, dense, dark- 

 colored membrane called the choroid H, which excludes all light 

 from the interior except what passes through the lens and iris. 

 This serves the same purpose in the eye as the black paint inside 

 the camera. 



The lens is "mounted" in a ring of muscles, called the ciliary 

 processes P, which focus the image by contracting or relaxing. 



