964 SEC. 18. BIOLOGY. 



the law of Listing, and the corresponding rotation. (Archiv fur 

 Ophthalmologie, XVI. 1, p. 154.) Prof. Dondcrs, Utrecht. 



In an outer fixed ring can be turned another ring, representing the prin- 

 cipal plane of axes, containing all the axes round which the eye moves out of 

 the primary into any secondary position. Before turning on the intended 

 axis, we put the arms of the cross vertical and horizontal ; after the rota- 

 tion these arms indicate the position of the vertical and horizontal meridians 

 of the eye. 



3975. Compound Phaenophthabno trope (large size, for 

 demonstration). Prof. Dondcrs, Utrecht. 



This instrument, as well as the simple phsenophthalmotrope, No. 3974, 

 demonstrates the movements of the eye according to the laAv of Listing ; but 

 can moreover put the line of fixation in any direction by rotation round a 

 vertical and a horizontal axis (Seitenwendung und Erhebung of He 1m- 

 holtz). The cross then indicates a position of the meridians different from 

 the position obtained by rotation round a single axis according to Listing's 

 law : the difference is the so-called "wheel-rotation" of Helmholtz. 



The same instruments (small size), for private study of the movements of 

 the eyes ; they enable us to compare the direction of the after- pictures with the 

 direction of the cross in corresponding movements. (Compare : Onder- 

 jjoekingen physiologisch laboratorium, Utrecht. 01. S. ii. D. iii. bl. 119, and 

 Archiv fur Ophthalmologie B. xvi. S. 160.) 



3976. Simple Phsenophthalmo trope, small size, for per- 

 sonal use. Prof. Dondcrs, Utrecht. 



3977. Compound Phsenophthalmotrope, small size, for 

 personal use. Prof. Dondcrs, Utrecht. 



3978. Snellen's Ophthalmotropometer, for determining 

 the movements of the eye to right and left. (Haudbuch der 

 Augenheilkunde von Saemisch und Graefe, III. p. 236.) 



Dr. Snellcn, Utrecht. 



This instrument is intended to determine the movements towards right and 

 left of each eye separately, and of both eyes with regard to each other. 



It consists principally of two bars rotating about one axis ; they form an 

 angle which can be read directly on a graduated arc, attached to one of the 

 bars. 



The instrument is to be placed so that the point of rotation of the bars is 

 perpendicularly below the point of rotation of one of the eyes. 



Eor this purpose, the plate upon which the bars lie can slide forward and 

 backward, and also to the right and to the left, independently of the points 

 against which the head rests with the infra-orbital margins. 



The eye under examination has to be fixed upon the middle of a telescope 

 carried by one of the bars. By moving the plate to right or left the instru- 

 ment is adjusted until the eye is in the middle of the field of view of the 

 telescope. Then the bar with the telescope is turned some degrees, and the 

 eye under examination has to be fixed again in the middle of the field of 

 view ; if the point of rotation of the eye does not coincide with the point of 

 rotation of the instrument, the eye will now be seen out of the middle of the 

 field of view. By sliding forward and backward until the eye appears again 

 in the centre of the field of view, the instrument is carried into the proper 

 position with regard to the eye. 



