580 KEl'ORT — 1891. 



Absolute Units of Measurement. By AV. Moox. 



The disadvantages of the C.G.S. system of units are that tlie units are so small 

 that one can form no conception of their value, and that owing to this smallness it 

 is necessary to introduce a separate set of practical units for ordinary purposes of 

 measurement. These disadvantages may be overcome by taking as fundamental 

 units L and M larger or T smaller. 



Of all the systems of metrical units that can be formed by varying L and T by 

 multiples or sub-multiples of ten, that system is the best that is founded upon the 

 ' Decimetre, Kilogramme, and Decisecond.' If the name 'Instant' were given to 

 the Decisecond, the system could be spoken of as the D.K.I, system. 



In the D.K.I, system ' g' = -981, or nearly unity, so that the weight of a kilo- 

 o-ramme could be taken as the unit of force for rough calculations. _ This would be 

 a great improvement, since the simplest way to conceive a force is as the weight 

 of unit mass. 



A prepared table shows that all the D.K.I, units are sufficiently large to be 

 used for practical purposes, and that all the multiples and sub-multiples of them 

 that would be required could be expressed by the usual Greek prefixes to their 

 names. 



To express the large numbers required for insulation, resistances, and the small 

 capacities of condensers, the Greek prefixes ' omega ' and ' omicron ' might be used 

 for powers of 10 and 10-' respectively. 



WEDyESDA Y, A UG UST 26. 



The following Papers and Report were read : — 



1. On the Measurement of Lenses. 

 By Professor SILVA^'t;s P. Thompson, F.L.8. 



The author described his focometer and some results obtained upon microscopic 

 objectives and camera lenses. Wide-angled lenses were found in all cases to have , 

 the positions of principal planes inverted. 



2. Oil a New Polariser. Bj Professor Silvanus P. Thompson, F.U.S. 



3. Some Exjperiments on a new Method for the Determination of ' v.' 

 By A. G. Webstee. 



The method is similar to that proposed by Maxwell with the title, ' Measure- 

 ment of a resistance in Electrostatic Measure.' A condenser is conaected m 

 parallel with the two sets of quadrants of an electrometer, and both are connected 

 in series with a battery and a high inductionless resistance. Contact being made 

 and broken after a short time t by means of a Helmholtz pendulum-interrupter, 

 the potential of the charge of the condenser and electrometer, measured by the 

 first swing of the latter, is 



7),j being the E.M.F. of the battery used, w the large resistance, c and y the respect- 

 ive capacities of the condenser and electrometer. A second set of experiments, c 

 being disconnected, gave the value of y, which included the capacity of the leading 

 wires and of an auxiliary condenser inserted for the purpose of making c + y and y 

 more nearlv equal. ■ 



