170 Sir D. Brewster 07i the Undulations excited in the Retina 



1. If we look through a narrow aperture, about the 50th of 

 an inch wide, at a bright part of the sky, or at the flame of a 

 candle, we shall observe the luminous ground covered with a 

 great number of broken parallel lines alternately light and 

 dark. These lines are always parallel to the narrow slit, and 

 of course change their place as the slit is moved round before 

 the eye. If we look through a number of parallel slits such 

 as the teeth of a comb, the broken parallel lines are seen more 

 distinctly: and if we give the comb a motion oblique to the 

 direction of its teeth, the broken lines become more distinct, 

 though less straight than before, arid new black lines appear 

 lyino- in different directions as if they were detached portions 

 of a number of dark ramifications. All these pha^nomena are 

 seen more distinctly when we look at homogeneous light ; but 

 I have not been able to perceive any marked difference of 

 magnitude in the spaces between the broken lines when they 

 are formed by differently coloured rays. 



If we use two systems of narrow slits and cross them at 

 different angles, we shall perceive two systems of broken lines 

 crossing each other at the same angles; and if when the lines 

 of the two systems are parallel we give one of them a rapid 

 alternating motion perpendicular to the direction of its slits, 

 the parallel broken fringes are seen with peculiar tlistinct- 

 ness. 



2. PliEenomena analogous to those now described, may be 

 seen by looking at a number of parallel black lines drawn 

 upon white paper, such as those which represent the sea in an 

 eno-raved map, or by looking at the luminous intervals be- 

 tween a number of parallel wires seen against the sky. If the 

 eye looks at an}' of these objects steadily and continuously, the 

 black lines soon lose their straightness and their parallelism, 

 and inclose luminous spaces somewhat like the links of a 

 number of parallel chains. When this change takes place, the 

 eye which sees it experiences a good deal of uneasiness, an 

 effect which is communicated also to the eye which is shut. 

 When this dazzling effect takes place, the luminous spaces be- 

 tween the broken lines become coloured, some with yellow, 

 and others with green and blue light. 



The phsenomena produced in these two experiments are 

 obviously owing to rectilineal undulations propagated across 

 the retina; and the interference and crossing of the undula- 

 tions, by which the dark lines are broken into detached pieces, 

 and by which the colours are produced, ai*ise from the un- 

 steadiness of the head or the hand, which causes a want of 

 parallelism in the successive undulations. 



3. The action of small and bright points of light upon the 



