THE OPTICAL DEFECTS OF THE EYE. 



Chapter I. — Optical Considerations. 



The eye is pre-eminently an optical instrument, and the phenomena 

 of vision all depend upon the laws of optics. Hence, a knowledge 

 of some, at least, of the elementary principles of light is essential to 

 a correct appreciation of the physiology of the eye. The diagnosing 

 of optical defects of the eye, — long and short sight, &c. &c., and 

 their treatment with the scientific use of spectacles, require some 

 knowledge of the laws of refraction, and the properties of convex and 

 concave lenses. 



The philosophy of the ophthalmoscope can hardly be understood 

 unless the principles of both refraction and reflection are thoroughly 

 mastered. 



You will therefore, I hope, not consider the time ill spent if, be- 

 fore proceeding with the investigation of diseases of the eye — you 

 review with me some of the elementary principles of optics which lie 

 at the foundation of all ophthalmic science. 



The nature of light is not known. I can no more tell you what 

 light is, than your professor of physiology can tell yon what life" is.. 

 We know that the sun shines, but how it shines we cannot tell. 



" Two difierent theories have been advanced of the more intimate 

 nature of light." " One, the Newtonian {corpuscular) conceives that 

 each luminous point is constantly giving ofi" a succession of luminous 

 corpuscles which follow each other in uninterrupted succession on an 

 imaginary line or axis like a string of beads on a rigid thread." 



The undulatory theory (Christian Huychens') on the other hand 

 considers space as pervaded by a subtle gaseous fluid or ether ; that 

 luminous bodies have the power of communicating to this ether a 

 wave motion which affects the retina the same as vibrations of the 

 air affect the auditory nerve. 



Sir John Herschel, speaking of the great ingenuity of the undula- 

 tory theory says, " if it is not true it deserves to be." 



The sun is the great natural source of light ; as it shines by its 

 own light it is called self-luminous. The fixed stars are also self-lu- 

 minous ; so is a lighted lamp and bodies in a state of ignition. But 

 most bodies by which we are surrounded, are seen only by reflected 



