VISION 185 



may be expected between the value of F determined from the above 

 formula and that determined directly. This deviation is due to errors 

 in the apparatus and in the observations. 



(4) Problems. The value of the formula - + - = - is so great 



and its application so frequent that the student should thoroughly 

 familiarize himself with the properties of lenses as revealed in this 

 formula. 



Solve the following problems: 



(1) When the object is twice the focal distance, what is the dis- 

 tance of the image? 



(2) When the distance of the object is greater than 2 F, how does 

 the distance of the image compare with 2 Ff 



(3) When the object is at a very great distance (o= oo), at what 

 distance will the image be formed? 



(4) What is the maximum focal distance that may be determined 

 or verified with the above-described apparatus? . 



Discuss in detail. 



V. TO LOCATE EXPERIMENTALLY IN THE MAMMALIAN EYE 



THE CARDINAL POINTS OF THE SIMPLE 



DIOPTRIC SYSTEM. 



In the study of the glass lens one takes into consideration the 

 index of refraction, and the radius of curvature of the surface of the 

 lens. When one remembers that the eye possesses media of two 

 different refractive indices, bounded by three curved surfaces 

 anterior corneal surface (radius 7.829 mm.), anterior and posterior 

 lens surfaces (radii 10 and 6 cm., respectively) the complexity of 

 the problem becomes apparent. 



It has been shown mathematically that a complex optical system 

 consisting of several surfaces and media, centered upon a common 

 optical axis, may be treated as if it consisted of two surfaces only. 



Applying this principle to the eye it has been found that the several 

 media and surfaces may be reduced to two parallel spherical surfaces 

 whose radii are 5.215 mm. These surfaces cut the optical axis just 

 posterior to the cornea and are only J mm. apart. 



To further simplify the optics of the eye it has been customary 

 to reduce it to a simple dioptric system by assuming one refracting 

 surface near the posterior surface of the cornea. 



A Simple Dioptric System. 



The simple dioptric system is one in which the ray passes from one 

 medium into a second medium of different refractive index, the 



