THE PRESIDENTIAL ADDRESS S 



viewed. Euclid in his optics perhaps used this as a mathematical 

 fiction practically equivalent to the modern one of reversing the 

 course of a ray, but other authors appealed to the apparent glow of 

 animal eyes by lamplight, which shows that they took the theory 

 quite literally. The Arabian author Alhazen had more correct ideas 

 and he gave an anatomical description of the eye, but apparently 

 regarded what we call the crystalline lens as the light-sensitive organ. 

 Kepler was the first to take the modern view of the eye. 



The detailed structure of the retina, and its connection with the 

 optic nerve, has required the highest skill of histologists in inter- 

 preting difficult and uncertain indications. The light-sensitive 

 elements are of two kinds, the rods and cones. The rods seem 

 to be the only ones used in night vision, and do not distinguish 

 colours. The cones are most important in the centre of the field of 

 view, where vision is most acute, and it seems to be fairly certain 

 that in the foveal region each cone has its own individual nervous 

 communication with the brain. On the other hand, there is not 

 anything like room in the cross-section of the optic nerve to allow 

 us to assign a diff"erent nerve fibre to each of the millions of rods. 

 A single fibre probably has to serve 200 of them. 



The nervous impulse is believed to travel in the optic nerve as 

 in any other nerve, but what happens to it when it arrives at the brahi 

 is a question for the investigators of a future generation. 



The use of lenses is one of the greatest scientific discoveries : 

 we do not know who made it. Indeed, the more closely we inquire 

 into this question the vaguer it becomes. Spectacle lenses as we 

 know them are a mediaeval invention, dating from about a.d. 1280. 

 Whether they originated from some isolated thinker and experi- 

 mentalist of the type of Roger Bacon, or whether they were developed 

 by the ingenuity of urban craftsmen, can hardly be considered certain. 

 There are several ways in which the suggestion might have arisen, 

 but a glass bulb filled with water is the most likely. Indeed, con- 

 sidering that such bulbs were undoubtedly used as burning glasses 

 in the ancient world, and that the use of them for reading small and 

 difficult lettering is explicitly mentioned by Seneca, it seems rather 

 strange that the next step was not taken in antiquity. Apparently 

 the explanation is that the magnification was attributed to the nature 

 of the water rather than to its shape. At all events, it may readily 

 be verified that a 4- or 5-inch glass flask full of water, though not very 

 convenient to handle, will give a long-sighted newspaper reader the 

 same help that he could get from a monocle. 



The invention of lenses was a necessary preliminary to the inven- 

 tion of the telescope, for, as Huygens remarked, it would require 

 a superhuman genius to make the invention theoretically. 



The retina of the eye on which the image is to be received has 



