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SUMMARY OF CURRENT RESEARCHES RELATING TO 



In the same way if a vibrating rod is viewed tkrough a hole in a 

 revolving disc, and the rate of revolution is varied until the period 

 coincides with that of the rod, the latter will always be seen in the 

 same phase and will appear stationary. 



The method may be applied to the analysis of many kinds of 

 periodic vibration, and to the examination of objects in motion. Dr. A. 

 van Beek used a revolving screen perforated with holes, by means of 

 which the object under the Microscope is periodically illuminated, to 

 estimate the rate of ciliary vibration in the cells of a frog's tongue. 



With such an apparatus it is not found possible to vary the 

 rate and constancy of the revolutions with sufficient delicacy, and 

 Herr Martius has consequently applied the electro-magnetic strobo- 

 scope, as used by Kronecker, to the same purpose. A strip of 

 paper (fig. 22) is made to vibrate between the source of light and the 



Fig. 22. 



diaphragm of the Microscope so that at each vibration the object is 

 illuminated by a flash of light. A great advantage is gained by 

 using a plain strip in place of a perforated screen ; for if the instru- 

 ment is so arranged that the strip while moving in one direction 



a collar fitted to the substage. The shaft, which carries a simple bar armature, 

 also carries upon its upper extremity a disc having two or four apertures, which 

 coincide with the apertures of the stage and substage two or four times during 

 the revolutions of the disc. The course of the current from the battery through 

 the instrument is through the spring touching the commutator, through the 

 shaft and frame of the instrument to the magnet. The speed of rotation can be 

 varied, experiment showing that the period of darkness should be to the period 

 of illumination about as three to one for the best efiects. 



