X. SPECIAL COLLECTIONS. 965 



The following can now be determined 



- 1. The mobility of each eye outward. 



2. The limits of the lateral binocular vision. 



3. The convergence of the two eyes in straightforward vision, and at 



any degree of lateral fixation. 



4. The degree of deviation in strabismus. 



5. The centre of motion of the eye. 



3979. Ophthalmo-statometer, for the determination of the 

 exorbital protrusion of the eye-ball. Dr. Snellen, Utrecht. 



Two rods, united by a cross rod, are supported on the inner and outer 

 margins of the orbit. From the cross rod a small crank can slide down until it 

 presses softly against the eye-ball covered by the eye-lid. The thickness of 

 the eye-lid is to be subtracted from the. reading. The position of the top of 

 the naked cornea may be read directly by means of a sight movable on ono 

 of the rods. (Handbuch der Ophthalmologie, Graefe und Saemisch III., 

 p. 199.) 



Manufactured by P. W. Hiele, Utrecht. 



3980. Instrument for the determination of the Movements 



of the Head and Eye, in varying directions of the line of fixation. 

 Donders and Ritzmann. (Compare Archiv fur Ophthalmologie, 

 XXI., 1, p. 131, 1875.) Prof. Donders, Utrecht. 



A bent wooden rod has on one end a jnouth piece, on the other an arc (with 

 a sliding sight), movable round an axis passing through the point of rotation 

 of the eye, which point is at the same time the centre of curvature of the arc. 



Over the axis of the arc the eye is fixed on a point a, then, without moving 

 the head, on a point 6, to which the sight is made to correspond ; now the eye 

 is again fixed upon a, and with free movement upon b ; from the required 

 displacement of the sight we learn how much the head has shared in the 

 movement. If after the free motion the plane of the arc should not pass 

 through b, we make it pass through this point, and read off how many degrees 

 we have had to turn. 



3981. Apparatus for verifying the Laws of Donders and 

 Listing (hitherto undescribed). Prof. Donders, Utrecht. 



A bent wooden rod has on one end a mouth-piece and on the other a 

 coloured strip, movable about an axis passing through the centre of move- 

 ment of the eye. 



After fixation in the direction of the axis, directing the eye upon ono 

 extremity of the strip, the after-picture is seen in the prolongation of the strip 

 (Law of Listing) in whatever way the line of fixation reaches the 

 extremity (Law of Donders). 



From experiments made with lateral inclination of the head, we may 

 learn how far the associated rotary motion interferes with the law of Listing. 



3982. Reflecting Lens, with a mouth-piece for autoscopy 



of the eye. Hollandische Beitrage zu den anatomischen und 

 physiologischen Wissenschaften, Diisseldorf und Utrecht, 1864, 

 p. 384 ; and Mulder, Archiv fiir Ophthalmologie, Bd. XXI., Abth. 

 ], 1875, Prof. Donders, Utrecht. 



