1884.] the state of an Eye affected with Astigmatism. 133 



la 1871, age 70, distance 1O0 inches; Reciprocal "100 



Difference -"029 

 In 1884, ... 83, 141 ; 071 



III. Measures of the astigmatic power of the left eye at 

 different epochs, estimated in each case by the differences of the 

 reciprocals of the same date in the two preceding tables. 



In 1825, age 24, astigmatism = '120 



•101 



•091 

 079 



•097 



Difference = - -019 



--010 



--012 



+018 



It appears probable that the measures in the second table are 

 somewhat erroneous, perhaps more particularly that for 1871. It 

 is not easy to make the observations at the greater distance. 



I am unable to give measures for the right eye corresponding 

 to those of preceding years. A defect, which has now existed for 

 many months, totally destroys vision in the centre of the field of 

 view. 



(2) On the measurement of the electrical resistance between two 

 neighbouring points on a conductor. By Lord Rayleigh, M.A. 

 (With an account of experiments by R. W. Shackle, M.A., and A. 

 W. Ward, B.A.) 



As an alternative to the method of Matthiessen and Hockin, 

 the following process may be used with good effect for the above 

 purpose. It is founded upon the combination of resistances de- 

 scribed in a paper upon the determination of the ohm by Lorenz's 

 method {Phil. Trans. 1883), by which a small effective resistance 

 is obtained from elements of moderate and accurately measurable 

 value. 



The main current 7 passes principally through a shunt of 

 resistance a, but partly through a small resistance b and a large 

 resistance c arranged in series. The terminals of a galvanometer 

 of resistance g are connected to the extremities of b. If g were 

 infinite, the difference of potentials at its electrodes would be 



ab 

 a + b+c 7 ' 

 so that abj{a + b + c) is the effective resistance of the combination. 

 For example, if a = 1, b = 1, c = 98, the effective resistance is jfa, 



