378 VISION. LECT. XX. 



the earth than the head. If an object presents itself to 

 us really reversed, relatively to the position in which we 

 are accustomed to see it, we consider that it has acquired 

 the reversed position because its image upon the retina is 

 equally so, by comparison, with that which we ourselves 

 hold, and to that in which we usually see it. We know that 

 a man, and that each of us, has his, or our feet upon the 

 ground : but when we see in the image formed upon the 

 retina by a dancer, that his head touches the ground, we 

 know that we see it in a reversed position. 



Judgment of Distance and Size. We judge of the dis- 

 tance and magnitude of objects in many ways : if they 

 were placed constantly at one distance, and always equally 

 illuminated, we should be able to measure their size by 

 that of the image painted on the retina. The dimension 

 of this image is, in general, proportional to the visual an- 

 gle made by two right lines drawn from the extremities of 

 the object to the centre of the retina : this we call the appa- 

 rent size. In judging of the distance of objects, we have 

 perception, 1st, of the movements which the eye makes in 

 order that the luminous cone, which the object sends to 

 the pupil, and which is more or less divergent according 

 to the distance, may form its focus upon the retina ; 2dly. 

 Of the movements by which we bring the optic axes of the 

 two eyes more or less near one another, in order to make 

 them converge upon an object placed at different'distances. 

 But this latter means of appreciating distances can no longer 

 serve us when the bodies are at great distances, for then 

 the two axes become almost parallel, and we are subject 

 to optical illusions. Thus, the two rows of trees of a 

 long avenue appear to approach nearer to each other in 

 proportion as they are further off ; and the lateral walls of 

 a long gallery, also, present this appearance. 



