334 



PRACTICAL PHYSIOLOGY. 



[LXXI. 



change in curvature of the lens, more especially of the anterior 

 surface, during accommodation (fig. 251). 



(a.) Place the phakoscope in a convenient position, and darken the room. 

 Two persons are required. The observed eye (patient) looks through a hole in 

 the box opposite to c, while the observer looks through the hole (a) at the side. 

 Light a lamp, place it some distance from the two prisms (b, b'} in such a 

 position that its light is thrown clearly upon the observed eye, and the 

 observer sees two small bright square images of light, when the observed eye 

 looks straight ahead at a distant object. These are the comeal images. He 

 should also see in the observed eye two larger less distinct images, from the 

 anterior surface of the lens, and two smaller much dimmer images, from the 

 posterior surface of the lens. The last are seen with difficulty. 



(b.) Ask the patient to accommodate for a near object, viz., the pin above c, 

 keeping the eye unmoved. Observe that the middle image becomes smaller 

 and goes nearer to the corneal one, while the other two undergo no perceptible 

 change. At the same time the pupil becomes smaller. 



FIG. 251. Phakoscope. a. Hole for observer's 

 eye ; 6, &'. Prisms ; c. Carries a pin for 

 the observed eye to fix as its near point. 



FIG. 252. Auber's Model to show 

 the principle of the Ophthal- 

 mometer. 



* 10. Principle of Helmholtz's Ophthalmometer. The student may con- 

 veniently learn the principle of this instrument from the apparatus of Auber 

 (fig. 252) (made by Petzold of Leipzig). By means of the ophthalmometer 

 Helmholtz measured the size of Sanson's images and the changes in size during 

 accommodation. If one looks at an object through a plate of glass in a direc- 

 tion at right angles to the surface of the glass, the object is seen single and in 

 its exact position. If, however, one looks at it obliquely or displaces the glass, 

 then the image appears displaced to the right or left according to the inclina- 

 tion of the glass plate. In Helmholtz's instrument two glass plates, as in fig. 



