X. SPECIAL COLLECTIONS. 967 



The instrument consists" of a head-holder and a frame-apparatus. In the 

 holder (on the principle of Heririg) as the head turns round the basal line as 

 axis, the eyes retain their places. The frame-apparatus can rotate round the 

 same axis, and consists of a fixed frame and two pairs of movable frames, one 

 pair for nearly vertical, one pair for nearly horizontal threads. 



One of the vertical or one of the horizontal threads may have a fixed posi- 

 tion ; the direction of the other is obtained by turning the frame, until both 

 the threads viewed with the eye appear to be parallel. The angle between 

 them in this position is read on the graduated arc with vernier. 



This instrument is used to determine the angle of the meridians of apparent 

 vertical and apparent horizontal direction, either separately or simultaneously, 

 at any degree of inclination of the plane of fixation, and at any degree of con- 

 vergence of the lines of fixation, either symmetrical or asymetrical, and to in- 

 vestigate,^ the same time, how lines or objects in the field of vision influence 

 these angles. 



3990. Perimeter Arc, with diagram of the plane of projec- 

 tion, for determining and registering the field of vision. (Hand- 

 buch der Augenheilkunde van Saemisch und Graefe, III. 

 p. 57.) Dr. Snellen, Utrecht. 



A little below the centre of a movable metallic semicircle is a point for the 

 infra-orbital margin to rest against ; thus the eye under examination is kept as 

 much as possible in the centre of the arc. . Behind the arc a black board is 

 placed, which is marked with meridians from 10 to 10, showing, as it were, 

 the projected arc with its divisions. The limits of the field of vision deter- 

 mined on the arc are mapped on the black board. 



The scheme obtained represents clearly the limits of vision on each 

 meridian. 



To be had from D. B. Kagenaar, Utrecht. Its price is 22-50 f. (I/. 185.) 



3991. Cycloscope of Donders and Kiister, to show the direc- 

 tion of the meridians, the great circles, the direction circles, and 

 the parallel circles of the field of vision, (Archiv fur Ophthal- 

 mologie, B. xxii. 1876.) Prof. Donders, Utrecht. 



This consists of a chair with head holder, and an arc with passing induc- 

 tion-sparks. The arc shows, by revolving round different axes, the direc- 

 tions of all meridians, all great circles, the direction circles of Helmholtz, 

 and parallel circles by sliding up and down a stand. When in a dark 

 room, the position of the head being well secured, one eye (the other being 

 covered) is placed in the centre of curvature of the arc and fixed, in the 

 primary position, upon a mark, which owing to its being rubbed with phos- 

 phorus throws out a faint light ; the induction sparks we may fancy to be 

 distant stars, and determine the direction in which they appear. On the 

 bearing of such investigations, compare Helmholtz, Physiologische Optik, 

 28, p. 550, and Kriister, /. c. 



3992. Horopteroscope, for the determination of the line- 

 horopter-plane, at different degrees of inclination of the plane of 

 fixation and at different degrees of convergence. 



Prof. Donders, Utrecht. 



The horopter-plane (a quadrangular plate) can revolve round the basal line 

 of the head fixed by the mouth-piece ; the relative distance of the two hori- 

 zontal stripes, seen with or without crossed visual lines, determines the 

 convergence ; the horot>ter-plane is now made to incline until the horizontal 



