oo6 



A MANUAL OP PHYSIOLOGY 



of the lens ; and a small inverted real image from the (concave) 

 posterior boundary of the lens (Purkinje-Sanson images). The 

 second image is intermediate in position between the other two. 

 It is possible with special care to make out a fourth image ; 

 but since it is reflected from the posterior surface of the cornea, 

 at which only a slight change in the refractive index occurs, it 

 is less brilliant than the first three. When the eye is accom- 

 modated for near vision, as in focussing the ivory point of the 

 phakoscope (Fig. 434), the corneal image is entirely unchanged 

 in size, brightness, and position. The middle image diminishes 

 in size, comes forward, and moves nearer to the corneal image. 



This shows that the curvature 

 of the anterior surface of the 

 lens has been increased that 

 is to say, its radius of cur- 

 vature diminished for the 

 size of the image of an object 

 reflected from a convex mirror 

 varies directly as the radius 

 of curvature. A slight change 

 takes place in the image from 

 the posterior surface of the 

 ]ens, indicating a small in- 

 crease of its curvature too. 

 By means of a method founded 

 on the observation of the 

 changes in these images, and 

 a special instrument called an 

 ophthalmometer which allows 

 of their measurement, Helm- 

 holtz has calculated that, 

 during maximum accommo- 

 dation, the radius of curva- 

 ture of the anterior surface 

 of the lens is only 6 mm., as 

 compared with 10 mm. when 

 the eye is directed to a distant 

 object and there is no accommodation. When the lens has been 

 removed for cataract, fairly distinct vision may still be obtained 

 by compensating for its loss by convex spectacles of suitable 

 refractive power (10 diopters* for distant vision, and 15 diopters 



* A diopter (i D.) is the unit of refractive power generally adopted in 

 measuring the strength of lenses, and corresponds to a lens of i metre 

 focal length. A lens of 2 diopters (2 D.) has a focal length of metre, a 

 lens of 4 diopters (4 D.) a focal length of % metre, and so on. The diverging 

 power of concave lenses is similarly expressed in diopters with the negative 

 sign prefixed. Thus, a concave lens of i metre focal length has a strength 

 of i D., and will just neutralize a convex lens of i D. 



FIG. 387. PURKINJE-SANSON IMAGES. 



A, in the absence of accommodation ; 

 B, during accommodation for a near 

 object. The upper pair of circles en- 

 close the images as seen when the light 

 falls on the eye through a double slit on 

 a pair of prisms ; the lower pair show 

 the images seen when the slit is single 

 and triangular in shape. 



