THE EYE. 



CHAPTEE III. 



76. Why objects are not seen double. 77. Exceptional cases. 



78. The eye has no direct perception of distance or magnitude. 



79. How distances are estimated. 80. Appearance of the sun and 

 moon when rising or setting. 81. How magnitudes are estimated. 

 82. Illusion produced in St. Peter's at Kome. 83. Magnitude 

 inferred from distance. 84. Perception of angular motion. 85. 

 How real direction of motion may be inferred. 86. Examples of the 

 sun and moon. 87-90. Effect of the motion of the observer 

 Examples. 91. Angular distance defined. 92. The eye has no 

 direct perception of form How inferred. 93. Visible area defined. 

 94. Figure inferred from lights and shadows. 95. Power of dis- 

 tinguishing colours. 96, 97. Absence of this power in particular 



76. THE question why, having two eyes on which independent 

 impressions are made by external objects, and on the retina of 

 each of which an independent picture of a visible object is formed, 

 we do not see distinct objects corresponding to each individual 

 external object which impresses the organ, is often asked. 



The first reflection which arises on the proposition of this 

 LARDNER'S MUSEUM OP SCIENCE. a 81 



No. 63. 



