154 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



cordingly. If we imagine it at A, it will look the size 

 of A, but if we imagine it at the distanced", it will seem 

 to be four times as large. If its real size and place is A", 

 and we imagine it to be at A, it will seem many times 

 too small. If, on the other hand, its real size and dis- 

 tance is represented as A and we imagine it at A", it will 

 look many times too large. For example, if we hold up 

 a finger before one eye (the other being shut), very 

 near to the eye, say an inch, its image completely covers 

 a large building one hundred yards distant. Now, if we 

 imagined the finger one hundred yards distant, it would 

 look as large as the building. 



The fact of dependence of apparent size on estimated 

 distance is well shown in the case of the sun and the moon. 

 We are accustomed to estimate the distance of terrestrial 

 objects, but have no means of judging of the distance of 

 celestial objects. Therefore different persons will differ 

 in the most extraordinary way about the apparent size of 

 the sun or the moon. Some will say that they look about 

 the size of a saucer, others the size of a dinner plate, 

 and others the size of the head of a barrel. There are 

 some extreme cases of persons who say they look about 

 the size of an orange, and others as big as a cart wheel. 



The mathematical relation between apparent size and 

 estimated distance is well shown by spectral images. 

 Look at the setting sun steadily for a moment. The 

 image of the sun is branded on the retina so strongly 

 that the brand remains for some time. Now, every 

 change in the retina, whether it be image or shadow 

 or brand, is seen as something in the field of view. 

 With the sun brand still on the retina, look where we 

 will — on the wall, on the floor, on the sky — we see a spec- 

 tral image of the sun. Now as to the size. Look on a 

 sheet of paper two feet off ; the image cast on the sheet 

 is about a quarter of an inch in diameter. Look at the 



