816 SEC. 18. BIOLOGY. 



This apparatus consists of (a) a head-holder, which, the head being fixed 

 in the primary position, can revolve round a horizontal axis perpendicularly 

 bisecting the basal line, and easily and quickly be secured in every position. 



(6.) At a distance of some meters a round disc with horizontal light line 

 of gas flames, situated in the principal plane of fixation, and possessing incisions 

 on the rim for stretching diameters. After fixing the eye for some time on 

 the middle of the light line, the gas is shut off, the head with the holder is 

 turned and the latter secured, when the after-picture coincides with the stretched 

 diameter. The difference between the inclination of the head aiid the inclina- 

 tion of the diameter is the rotary motion. 



3986. Volkmann's Discs, united on one plate for the deter- 

 mination of the angle between meridians of apparent equal direc- 

 tion. The distance between the two axes must be equal to the 

 distance of the parallel visual lines of the experimenter. 



Professor Donders^ Utrecht. 



3987. Volkmann's Discs of Glass, facilitating parallel 

 position of the lines of fixation by looking at a distance. 



Professor Donders, Utrecht. 



3988. Stereoscope with revolving mirrors for the deter- 

 mination of the angle between the meridians of apparent equal 

 direction at different degrees of convergence. 



Professor Donders, Utrecht. 



3989. Isoscope, to determine the angle of the meridians of 

 apparent vertical and apparent horizontal direction. (Onder- 

 zoekingeu physiologisch laboratorium, Derde Reeks, 111. 2., 45 ; 

 and Archiv fur Ophthalmologie, B. XXL 3. S. 100.) 



Prof, Donders, Utrecht. 



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

 holder (on the principle of Hering) 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, at 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. Behind the arc a black board is 



