6 



Mr. J. H. Poynting on a method of 



[Nov. 21, 



desirable, each reflection at the mirror, AB, adding 20 to the deflection 

 of the ray. I have, for instance, employed three reflections from the 



beam-mirror, so multiplying the deflection six times. In this case, 

 one division of my scale, at the distance at which it was placed from 

 the beam, corresponded to a deflection of 7" in the beam, and this 

 conld be subdivided to tenths by the eye. The only limit to the 

 multiplication arises from the imperfection of the mirrors and the 

 decrease in the illumination of the successive reflections.* 



The chair of the observer is placed on a raised platform, and a small 

 table rising from the platform and free from the frame on which the 

 instruments rest, is between the observer and the telescope. On this 

 he can rest his note-book during an experiment. As the differences of 

 weight observed are sometimes exceedingly minute, the balance is made 

 very sensitive — usually vibrating in periods between 30" and 50". The 

 value of a division of the scale cannot be determined by adding known 

 small weights to one pan, as the deflection would usually be too great. 

 Any approach of the observer to the caso causes great disturbances, so 

 that the ordinary method of moving a rider an observed distance 

 along the beam is inapplicable. In some experiments made last 

 year I calculated the force equivalent to the small differences in 

 weight, in absolute measure, by observing the actual angular deflec- 

 tion and the time of vibration. With a knowledge of the moment 

 of inertia of the beam and treating it as* a case of small oscillations, 

 it was possible to calculate the value of the scale. But the observa- 

 tions and subsequent calculations were so complicated that the 

 following' method of employing riders was ultimately adopted. 



A small bridge about an inch long (fig. II, 1) is fitted on to the beam. 

 The sides of the bridge are prolonged about half an inch above the 



* This method was used in the seventh and eighth series here recorded. Two 

 reflections from the beam mirror were employed, giving four times the actual deflec- 

 tion. 



