THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



* AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



NOVEMBER 1853. 



XL VIII. On the Methods of Measuring very Small Portions of 

 Time, and their application to Physiological Purposes. By 

 Professor Helmholtz*. 



IT is my intention today to direct the attention of the meet- 

 ing to a series of experiments with which I have lately 

 been occupied, and the object of which is to ascertain, by means 

 of the refined methods of measurement furnished by modern 

 physics, the duration of certain quickly-passing processes which 

 occur in the living body. In the first place, I may be per- 

 mitted, so far as it is necessary to the understanding of what 

 is to follow, and so far as it is possible, without demonstration 

 on the apparatus itself, to describe the means by which it is ren- 

 dered possible to measure as small fractions of a second of time 

 as those into which, by the help of the most powerful micro- 

 scopes, our smallest unit of space, the line, may be divided. 



The perception of small differences of time by means of our 

 senses and without artificial help is not very fine, especially when 

 the processes whose difference of time is to be determined are 

 made the objects of different organs of sense, for example of the 

 eye and ear, or even of different parts of the same organ, for 

 example, of the eye* in various portions of the field of view. 

 The remarkable fact discovered by Bessel in the case of dif- 

 ferent individuals observing the transit of stars may be men- 

 tioned here. The observing astronomer perceives by the ear the 

 tick of the clock, at the same time he sees the motion of the star 

 towards the wire of the telescope, and endeavours to mark the 

 positions of the star at the last tick before crossing, and at the 



* Read before the Physico-Agricultural Society of Koxiigsberg ; com- 

 municated by Dr. Tyndall. 



Phil. Mag. S. 4. Vol. 6, No. 40. Nov. 1853. Y 



