962 SEC. 18. BIOLOGY. 



The refraction in the principal meridians changes with the rotation propor- 

 tionately to the sines of the angle between the axes of the cylinders. The 

 scale which is to be read has been constructed on the rim o one of the 

 glasses. For the determination of the refraction, parallel lines are viewed 

 through this lens of changeable power, the lines are perpendicular to one of 

 the principal meridians, and placed either at a distance or united with the lens 

 in a tube. Manufactured by A. Cretes, optician, Paris. 



797. Set of Spherical Lenses , metrical system (by Roulot). 



Prof. Donders, Utrecht. 



3965. Cylindrical Glasses (simple cylindrical, bi-cylindrical, 

 and spherico-cylindrical) introduced by Donders (Astigmatismus 

 en cylindrische glazen ; Utrecht, 1862). Metrical System by 

 Roulot. Prof. Donders, Utrecht. 



To detect and correct astigmatism. By turning a very weak cylindrical 

 glass before the eye iu a plane perpendicular to the visual line, it will be 

 found that no eye is absolutely free from astigmatism. 



3966. Test-types, for the determination of the acuteness of 

 vision. Williams and Norgate, London. Dr. Snellen, Utrecht. 



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 normal visual angle, and the 



sight is expressed by V = , d being the distance at which the letters are 



recognised, and D the distance at which they exhibit themselves under an 

 angle of five minutes. 



3967. Set of Prismatic Glasses (by Roulot), introduced 

 by Donders. Prof. 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 tho 



right and left eye. 



f. The actual difference of those images in anisometropia. 



g. 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 



