966 SEC. 18. BIOLOGY. 



Principally used to enable the eye, looking at a vessel of the conjunctiva, 

 to observe its own rotations associated with lateral inclinations of the head. 

 (The first application of the method consisting in uniting an instrument witk 

 the head, in order to secure the relative direction of the line of fixation.) 



3983. Mouthpiece, witli two vertical rods. Showing 

 the rotations associated with lateral inclinations of the head and 

 trunk, to be constantly equal for both eyes. 



Prof. Donders, Utrecht. 



The rods parallel to the apparent vertical meridians, at a distance of 70 mm., 

 produce parallel half images near each other. In all movements of 

 the head the images remain parallel, thus showing the rotations associated 

 with lateral inclinations of the head and trunk, to be constantly equal for 

 Hoth eyes. 



3984. Rod, with graduated arc to determine the degree of 

 lateral inclination of the head. Prof. Donders, Utrecht. 



Seen with parallel visual lines, the rod shows double images of equal direc 

 tion with the head (to be used in experiments of parallel rotations, &c.) 



3985. Apparatus for determining the rotary motion connected 

 with temporary and permanent lateral inclination. Donders and 

 Mulder (Archiv. far Ophthalmologie, XXI. Abth. 1, p. 68, J875.) 



Prof. Donders, Utrecht. 



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. 



(&.) 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 and 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. 



Prof. DonderS) Utrecht. 



3987. Volkmann's Discs of Glass, facilitating parallel 

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



Prof. 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. 



Prof. Donders, Utrecht. 



3989. Isoscope 9 to determine the angle of the meridians of 

 apparent vertical and apparent horizontal direction. (Onder- 

 zoekingen physiologisch laboratorium, Derde Reeks, III. 2., 45 ; 

 and Archiv fiir Ophthalmologie, B. XXI. 3. S. 100.) 



Prof. Donders, Utrecht. 



