348 PROCEEDINGS OF THE AMERICAN ACADEMY. 



upon each point of the surface of the animal ; the intensity of the light 

 from any single square centimeter of the area is only 0.0025 CM., but 

 since there are 1(J,000 such radiating squares the total intensity is 

 25 CM. 



In an animal possessing eyes capable of forming good images of ex- 

 ternal objects the conditions are very different. In this case light from 

 all parts of the large area cannot fall upon every point in the sensitive 

 surface (here the retina), but the light from each part of the field re- 

 tains its position relative to that from other parts, and falling upon the 

 retina in this order covers there an area similar in shape and relative 

 intensity of illumination to the external one. Such is the image. The 

 small light, which we are considering as only 1 cm. square, would 

 likewise form an image on the retina, and this, with the light at the 

 given distance, would cover a certain small area, which we may denote 

 by X. X, then, is the size of the retinal image of a luminous object 

 1 cm. square at a distance of 2 meters. The light received on this area 

 would have a certain intensity, which may be designated by y. Now, 

 since the large light has 10,000 times the area of the smaller one, its 

 image on the retina (making no allowance for aberration or other optical 

 defects) would be 10,000 times as large as the image of the small light, 

 or 10,000 X ; and the intensity on any single area x would be only 

 iwoo y- It is obvious, therefore, that under these circumstances we 

 have entirely different conditions of stimulation on the two sides — 

 that is, in the two eyes — of the animal. On the retina of one eye only 

 a very small area (.r) is stimulated, but the light has a considerable 

 intensity, which we have called y. The retina of the other eye is 

 stimulated over a much larger area (10,000 x), but each area (.r) re- 

 ceives in this case a light intensity of only towo y- 



This is of course only an approximation to the relative influence of 

 the two sources of illumination. It is probable that even in the most 

 highly organized eyes, owing to aberration and other defects, the actual 

 conditions are far from those here assumed. It must be remembered, 

 furthermore, that fropa a physiological standpoint the retina cannot be 

 accurately divided into areas, such as we have assumed ,r to be, corre- 

 sponding to external areas, but that its physiological action, as well as 

 its finer structure, must be considered in terms of visual elements — 

 the ommatidia in the compound eye, the rods and cones in the verte- 

 brate eye. The matter is further complicated by the fact that these 

 visual elements may not have a uniform distribution over the whole ret- 

 ina, nor do we know that those of different parts of the organ are equally 

 sensitive to light stimulation. 



In spite of these defects and deviations from the suppositional case, 



