THE EYE AS AN OPTICAL INSTRUMENT 



225 



If, therefore, a person be taken into a dark room and a candle be 



held on one side of his eye while he looks at a distant object, an 



observer can see three images of the flame in his pupil, due to 



that portion of the light reflected from the surfaces between the 



media. One image (a, Fig. 88) is erect and 



bright, reflected from the convex mirror formed 



by the cornea; the next, b, is dimmer and also 



erect; it comes from the front of the lens. The 



third, c, is dim and inverted, being reflected from 



the concave mirror (see Physics) formed by the 



back of the lens. When the curvature of a 



curved mirror is altered the size of the image a ges IG 'of 8 a7lance- 



reflected from it is also altered, becoming small fl^J| d fr 0m e the It 



when the radius of curvature of the mirror is fracting media of 



lessened and vice versa. If the three images be 



carefully watched while the observed eye looks at a near object 



in the same line as the distant point previously looked at, it is 



seen that the image due to corneal reflection remains unchanged ; 



that due to light from the front of the lens becomes smaller and 



brighter; the image from the back of the lens also becomes very 



ch 



FIG. 89. Diagram to illustrate the mechanism of accommodation; on the 

 right half of the figure for a near, on the left for a distant, object; rf, ciliary mus- 

 cle; ch, ciliary process of choroid; si, suspensory ligament; i, iris. 



slightly smaller. The change in the curvature of the front of the 

 lens can be calculated from the change in size of the image re- 

 flected from it when the eye changes from distant to near ac- 

 commodation. When a distant object is looked at the radius of 



