752 



THE PHAKOSCOPE. 



tion. Helmholtz, with the aid of the ophthalmometer, has measured the diminution of the 

 radius of curvature of the anterior surface of the lens during accommodation for a near object 



[Phakoscope. These images maybe readily shown by 

 means of the phakoscope of v. Helmholtz (fig. 536). It 

 consists of a triangular box with its angles cut off and 

 blackened inside. The observer's eye is placed at a, while 

 . on the opposite side of the box are two prisms, b, 6 1 ; 

 9 the observed eye is placed at the side of the box opposite 

 to C. When a candle is held in front of the prisms, b 

 and b\ three pairs of images are seen in the observed 

 eye. Ask the person to accommodate for a distant ob- 

 ject, and note the position of the images. On pushing 

 up the slide C with a pin attached to it, and asking him 

 to accommodate for the pin, i.e., for a near object, the 

 position and size of the middle images chiefly will be 

 seen to alter as described above.] 



2. In consequence of the increased curvature of the 

 lens during accommodation for a near object, the re- 

 fractive indices within the eye must undergo a change. 

 According to v. Helmholtz the annexed measurements 

 obtain in negative and positive accommodation respec- 

 tively. 



3. Lateral View of the Pupil. If the passive eye be 

 looked at from the side, we observe only a small black 

 strip of the pupil, which becomes broader as soon as the 

 person experimented on accommodates for a near object, 

 as the whole pupil is pushed more forwards. 



4. Focal Line. If light be admitted through the cor- 

 nea into the anterior chamber, the " focal line " formed 

 by the concave surface of the cornea falls upon the iris. 



If the experiment be made upon a person whose eye is accommodated for a distant object, so 

 that the fine lies near the margin of the pupil, it gradually recedes towards the scleral margin 



Fig. 536. 

 Phakoscope of Helmholtz. 



of the iris, as soon as the person accommodates for a near object, because the iris becomes more 

 oblique as its inner margin is pushed forward. 



5. Change in Size of Pupil. On accommodating for a near object, the pupil contracts, 

 while in accommodation for a distant object, it dilates (Descartes, 1637). The contraction takes 

 place slightly after the accommodation (Donders). This phenomenon may be regarded as an 

 associated movement, as both the ciliary muscle and the sphincter pupillre are supplied by the 

 oculomotorius ( 345, 2, 3). A reference to fig. 533 shows that the latter also directly supports 

 the ciliary muscle ; as the inner margin of the iris passes inwards (towards r), its tension tends 

 to be propagated to the ciliary margin of the choroid, which also must pass inwards. The 

 ciliary processes are made tense, chiefly by the ciliary muscle (tensor choroidal). Accommoda- 

 tion can still be performed, even though the iris be absent or cleft. 



6. Internal Rotation of the Eye. On rotating the eyeball inwards, accommodation for a 

 near object is performed involuntarily. As rotation of both eyeballs inwards takes place when 

 the axes of vision are directed to a ticar object, it is evident that this must be accompanied 

 involuntarily by an accommodation of the eye for a near object. 



7. Time for Accommodation. A person can accommodate from a near to a distant object 

 (which depends upon relaxation of the ciliary muscle) much more rapidly than conversely, from 

 a distant to a near obiect ( Vierordt, Aeby). The process of accommodation requires a longer 

 time, the nearer the object is brought to the eye (Vierordt, V bickers and Hensen). The time 



