812 SEC. 18. BIOLOGY. 



With letters and figures of definite magnitude are given, in metrical mea- 

 sure, the distances at which they exhibit themselves under an angle of 

 five minutes. This angle is assumed to be the smallest normal visual angle, 



and the sight is expressed by V = -., d being the distance at which the letters 



are recognized, and D the distance at which they exhibit themselves under 

 an angle of five minutes. 



3967. A set of Prismatic Glasses (by Roulot), introduced 

 by Donders. P r f* Donders, Utrecht. 



Besides their application in investigating and compensating anomalies of 

 the eye, they are used to demonstrate : 



a. The influence of the tendency to maintain binocular single vision on the 



movements of the eye. 



b. The maximum of divergence of the visual lines. 



c. The faculty of equal accommodation at different degrees of convergence. 



d. The apparent angle between equally directed meridians of the two- 



eyes. 



e. The want of local sign (Local-zeichen) to distinguish the images of the 



right and left eye. 



f. The actual difference of those images in anisometropia. 

 y. Pseudoscopy at double vision of horizontal lines. 



3968. Microscope, to measure the depth of the chamber of 

 the eye. Prof. Donders, Utrecht. 



By sliding the whole tube of the microscope we consecutively adjust for a, 

 surface of the cornea, b iris, and c, reflex image ; the reading from a to b is 

 the apparent depth of the chamber, the reading from a to c furnishes the 

 radius of curvature of the cornea, which enables us to calculate the real depth 

 from the observed apparent depth. On the middle of the objective glass is 

 pasted a very small piece of mirror : the eye observed, on looking in this 

 mirror at the reflex image of a distant flame, is properly directed, and accom- 

 modated for distance, and shows to the observer the reflex image for the 

 determination of the radius of curvature of the cornea. 



3969. Fhacoidoscope. (Anomalies, &c., p. 16.) 



Prof. Donders, Utrecht. 



A modification of the so-called ophthalmoscope, by means of which 

 Cramer discovered the changes of form of the crystalline lens to be the 

 true principle of accommodation. (Het accommodatie vermogen physio- 

 logisch toegelicht. Haarlem. 1853.) The three reflex images are seen in the 

 eye under changeable angles, as the eye accommodates itself alternately to a 

 distant and a near object. 



3970. Meridian-ring of the Ophthalmometer. (Anoma- 

 lies, &c., p. 361.) Prof. Donders, Utrecht. 



A flat ring, 388 mm. diameter, upon which three small lamps can be 

 moved; their reflexion -images have been used by Dr. Middelburg for the 

 determination of the radius of curvature of the cornea in all meridians. 



3971. Stenopaeic Apparatus and narrow slit. (Compare 

 van Wyngaarden, Archiv fiir Ophthalmologie, I. 1, p. 251.) 



Prof. Donders, Utrecht. 



