INTRODUCTION TO OPTICS. 



Ixxix 



proceeding from the lower part of an object fall upon the upper part of 

 the retina ; but as we know their direction to be from below, we see that 

 part of the object they describe as the lowest. When you wish to see 

 an object above you, you look upwards ; when an object below, you look 

 downwards. You look up to see an elevated object, for it is only thus 

 that the rays which proceed from it fall upon the retina of your eyes, 

 and they must do so if you are to see the object ; but the very circumstance 

 of directing your eyes upwards convinces you that the object is elevated, 

 and teaches you to consider as uppermost the image it forms on the retina, 

 though it is in fact represented in the lowest part of it. When yon look 

 down upon an object, you draw your conclusion from a similar reason- 

 ing. It is thus that we see all objects in the direction of the rays which 

 reach our eyes. 



The different apparent dimensions of objects at different distances pro- 

 ceed from our seeing, not the objects themselves, but merely their image 

 on the retina. Fig. 8 represents a row of trees, as viewed in the camera 



Fig. 8. 



obscura ; the direction of the rays from the objects to the image is ex- 

 pressed by lines. Observe that the ray which comes from the top of.the 

 nearest tree, and that which comes from the foot of the same tree, meet 

 at the aperture, forming an angle of about twenty-five degrees : this is 

 called the angle of vision, being that under which we see the tree. These 

 rays cross each other at the aperture, and represent the tree inverted in 

 the camera obscura. The dimensions of the image are considerably 

 smaller than those of the object, but the proportions are perfectly pre- 

 served. The upper and lower ray, from the most distant tree, form an 

 angle of not more than twelve or fifteen degrees, and an image of propor- 

 tional dimensions. Thus two objects of the same size, as the two trees 

 of the avenue, form figures of different sizes in the camera obscura, 

 according to their distance, or, in other words, according to the angle of 

 vision under which they are seen. 



The experience we acquire by the sense of touch corrects the errors 

 of our sight with regard to objects within our reach ; we are so perfectly 

 convinced of the real size of objects which we can handle, that we do not 

 attend to their apparent difference. The opposite house does not appear 

 to you much smaller than if you were close to it ; and yet you see the 

 whole of it through one of the windows of the room you sit in, and the 

 image of the house on your retina must be very considerably smaller than 

 that of the window through which you see it. Those accustomed to draw 

 from nature are well aware of this difference. When we look up an 

 avenue, the trees not only appear smaller as they are more distant, but 

 seem gradually to approach each other till they meet in a point, for the 



