PHYSIOLOGICAL OPTICS 795 



These vibrations in ether are propagated at an almost inconceivably rapid rate. 

 If we reckon the distance of the earth from the sun at 91,500,000 miles, the speed 

 of sunlight may be calculated at 185,500 miles in a second. Thus, it would require 

 this light about eight minutes to reach the earth, that of Neptune about four 

 hours, that of Centaurus close to 4 years and that of Sirius 17 years. Light 

 therefore, travels with a velocity which is 900,000 times greater than that 

 of sound; moreover, its stimulating power is extremely great, because a flash of 

 lightning, lasting H > o o o j o o o sec. , suffices to produce a visual sensation. SunHght, 

 of course, is the strongest light, equalling the power of 5500 candles placed at a 

 distance of one foot. It is 600,000 as strong as the reflected light of the moon 

 and 16,000,000,000 as strong as that of Centaurus. 



In passing away from its source, light is brought into contact with different 

 bodies, which tend to hinder its course. Only the most perfect vacuum allows 

 it to pass with as much freedom as the air. Other media are classified as trans- 

 parent, translucent and opaque. Transparent media permit of the passage of 

 white light, as well as of its spectral components, so that any object may be seen 

 through them in its different colors. Among these might be mentioned the air, 

 water, glass, the humors of the eye, and others. Translucent bodies allow only a 

 certain number of the light rays to pass, so that a clear outline of the objects 

 cannot be obtained. Opaque bodies cannot give rise to visual sensations, because 

 they prevent the passage of these rays, although permeable to them. The light is 

 then said to be absorbed, i.e., it is converted into some other form of energy, such 

 as heat. 



Reflection. — ^Luminous bodies are those which emit light, such as 

 the sun or substances when undergoing combustion. A luminous ray, 

 therefore, may be defined as the direction of the line in which light is 

 propagated, and a pencil of light as a collection of rays from the same 

 source. In this form, it consists of a number of divergent rays, i.e., 

 of rays which in passing away from the luminous object, gradually 

 become more widely separated from one another. A beam of light 

 includes a large number of light rays showing measurable dimensions. 

 It embraces divergent, parallel and convergent rays, but thereon vergent 

 rays are of no use to us under ordinary conditions. 



If light is made to pass through a homogeneous medium, such as 

 air, glass or water, it is propagated onward in a right line, while if an 

 opaque body is placed in its path, it will be intercepted by it and 

 be absorbed or reverberated. In the latter case, the light is forced to 

 change its direction, although allowed to continue onward in the same 

 medium. To this phenomenon the name of reflection has been given. 

 Reflecting bodies may be polished or unpolished. The first give rise 

 to regular and the second to diffused reflection. Thus, if a beam of 

 light is incident upon a well-polished mirror, the greater part of the 

 light is reflected in a single direction at a perfectly definite angle; 

 in fact, the reflection is so precise that it may be said to be governed 

 by two laws, as follows: 



(1) The reflected ray BE is in the plane of the incident ray DB and a normal or 

 perpendicular AB erected upon the reflecting surface CF at the point of incidence 

 B of the ray (Fig. 404). 



(2) The angle EBA formed by the reflected ray and the perpendicular, equals 

 the angle DBA made by the incident ray and the perpendicular. In other words, 

 the angle of reflection is equal to the angle of incidence. 



