X. SPECIAL COLLECTIONS. 813 



3972. Metallic Plates with two openings, for Entoptic 



Observation. (Anomalies, &c., p. 201.) 



Prof. Bonders, Utrecht. 



To determine the position as to depth of entoptic objects (muscae voli- 

 tantes, black spots, pearly spots, &c.) in the humours of the eye, by the 

 method " a double vue." Those of one eye are projected in the visual field 

 of the other, and the distance of the double images measured with a pair of 

 compasses (sliding compasses). 



3973. Optometer. To determine the relative range of accom- 

 modation. (On the anomalies of accommodation and refraction of 

 the eye. New Sydenham Society, London, 1864, p. 115.) 



Prof. Danders, Utrecht. 



An oblong quadrangular board on a stand. The board possesses three 

 grooves with scales, in which a wire optometer or a point can be moved. On 

 the width of the board are two half-rings supporting the glasses, whose 

 relative distance may be regulated by means of screws. Each ring is mov- 

 able in a circular groove, the centres of the circles coinciding with the centres 

 of motion of the eyes. The position of the eyes can be controlled by small 

 microscopes fixed on the sides of the board, and remains secured by the 

 cheeks resting against two wooden rods. By this contrivance, the distance 

 between the glass and the eye remaining unchanged, the visual line may at 

 any degree of convergence coincide with the axis of the glass. This instru- 

 ment is used to determine the play of accommodation at every degree of 

 convergence, i.e., curves of the nearest and farthest points, as functions of the 

 convergence (relative range of accommodation). 



3974. Simple Phsenophthalmotrope (large size for de- 

 monstration), to demonstrate movements of the eye according to 

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

 Ophthalmologie, XVI. 1, p. 154.) Prof. Donders, 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 Phsenophthalmotrope (large size, for 

 demonstration). Prof. Donders, Utrecht. 



This instrument, as well as the simple phsenophthalmotrope, No. 30, 

 demonstrates the movements of the eye according to the law 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 Helm- 

 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- 

 zoekingeu physiologisch laboratorium, Utrecht. Cl. S. ii. D. iii. bl. 119, and 

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



