186 Messrs. E. Marsden and H. Richardson on the 



platform P and adjusted to any required distance from the 

 zinc-sulphide screen Z by means of a ratchet-screw, S^ 



Fig. 1. 



The experiments were carried out in a dark room. The 

 microscope, along with the zinc-sulphide screen, was raised 

 until the screen was just outside the range of the a particles, 

 this adjustment being made by observation of the scintilla- 

 tions. Only the a particles given off from the RaC deposited 

 on the inner surface of the mica were used in this adjustment, 

 as they have the longest range. Owing to their large number 

 and homogeneity, however, the adjustment could be made 

 rapidly and accurately and the position afterwards read on 

 the vernier and scale, V. After the readings corresponding 

 to several adjustments had been taken, the metal foil was 

 placed in position and the end of the range again determined. 

 The difference between the readings then gave the air- 

 equivalent of the foil in the particular position. The 

 position of the foil was then altered and recorded on the 

 scale K, and the adjustment to the end of the range 

 repeated. In ithis way a series of values of the air- 

 equivalent was obtained for different positions of the foil. 

 To obtain an average value in any position, the foil was 

 moved in a horizontal plane and readings taken for different 

 positions. 



The following table gives an example of a set of 

 readings:— 



