X. SPECIAL COLLECTIONS. 811 



3962. Isochronoscope. Prof \ Bonders, Utrecht. 



A brass lever has at one extremity a registering spring and at the other 

 extremity a peg. On pressing the lever down with the hand, the spring re- 

 gisters the instant at which the peg, touching the mercury, closes the circuit 

 (white electrodes). At this instant, the physiological or psycho-physical 

 effects of this currrent, in all kinds of experiments of irritation, are registered 

 on the same cylinder. Underneath the lever are two electro-magnets. 

 When the current .(green electrodes) passes, the lever is pulled down. 

 This current is used either to note certain periods of time, as seconds, on 

 the cylinder, or, by means of a more equal closing of the circuit, to obtain 

 greater equability of the irritating current than can be obtained by the hand. 

 The equable breaking of the circuit is then secured by means of a spring 

 under the lever. 



3963. Apparatus for the Determination of the Co- 

 efficient of Elasticity of the Living Muscles in Man. 



(Compare Mansvelt, Elasticiteit der Spieren. 1860. Diss. inaug. 

 Utrecht, 1863.) Prof. Bonders, Utrecht. 



A vertical wooden board, sliding up and down in a stand with foot, has 

 on its upper side a little sliding beam, and on its surface two recesses, which 

 are the centres of two graduated arcs. With the face directed towards the 

 profile of the board, the operator leans with the shoulder against an extremity 

 of the cross beam and softly presses the internal condyles of the humerus into 

 one of the recesses. The humerus has a vertical position ; the forearm is 

 stretched either horizontallyat 0, or above or below this position, and a weight 

 suspended by a thread to a band round the wrist. On cutting this thread the 

 forearm springs up, and the number of degrees is read off. 



From different data the length of the muscle before and after the cut- 

 ting of the thread and the action of the weight may be calculated, whence 

 may be inferred the lengthening of certain muscles by definite weights at 

 definite degrees of fatigue. One edge of the board is designed for the experi- 

 ment with the left, the other for the experiment with the right, arm. 



3964. The Double Spectacles, for the determination of 

 refraction (Handbuch der Augenheilkunde von Saemisch und 

 Graefe, III., p. 50). Dr. Snellen, Utrecht. 



~ This is an opera-glass in the form of a pair of spectacles, consisting of a pair 

 of negative glasses of one inch focal distance, and in front of these a pair of 

 positive glasses of two inches focal distance. On changing the distance of the 

 positive and negative glasses a hand is made to move on a disc furnished with 

 a scale, which indicates the refraction of the system corresponding with the 

 different degrees of sliding. To be had from R. Jung, optician, Heidelberg. 



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

 and spherico-cylindrical) introduced by Bonders (Astigmatismus 

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

 Roulot. Prof. Bonders, Utrecht. 



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

 glass before the eye in 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. 



