X. SPECIAL COLLECTIONS. 817 



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

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

 mined on the arc are mapped on the blackboard. 



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/. 18s.) 



3991. Cycloscope of Bonders and Kriister, 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, of all great circles, of the direction circles of Helmholtz, 

 and, by sliding up and down a stand, of parallel circles. 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 horopter-plane is now made to incline until the horizontal 

 stripes have one direction, and then turned round a horizontal axis passing 

 through these stripes until the two perpendiculars on the first stripes are also 

 parallel. (By this contrivance we find the position for close work with the 

 hands, which, according to Donders, has in course of time determined the 

 angle between meridians of apparent equal direction.) 



3993. Spark-Stereoscope, with coloured lenses, showing 

 stereoscopic vision independent of movement of the eye. Archiv 

 fiir Ophthalmologie, Band xiii. p. 33. Prof. Donders, Utrecht. 



A box with two apertures for the eyes. A series of very small induction- 

 sparks is used as object of fixation. Between two electrodes attached to the 

 sliding lid of the box, we may cause a large spark to pass at different dis- 

 tances before and behind the object of fixation. In monocular vision we fail 

 in recognising the place of the large spark with regard to the object of fixa- 

 tion ; but in binocular vision, at the first spark we judge correctly. Between 

 the apertures for the eyes are other electrodes. The spark passing between 

 these last two electrodes is out of sight, but its two reflex images are seen 

 on a coloured lens ; one image is coloured the other not. In binocular vision, 

 their- position with regard to the object of fixation is soon distinguished ; but 

 without an object of fixation, many sparks are required for accurate estima- 

 tion. In general such coloured lenses are very convenient for demonstration 

 and mutual comparison of the reflex images of concave and convex surfaces. 



3994. Photostereoscope, to determine the relative value of 

 the second eye for binocular vision, constructed upon the principle 

 of Hering. Prof. Donders, Utrecht. 



39508. 3 F 



