858 Sir E. Rutherford and Dr. E. N. da C. Andrade on 



purposes of measurement to mark its position relative to the 

 holder. To enable this to be done, a narrow strip was 

 removed from this black paper, and the hole thus formed 

 covered with a slip of metal in which was a fine slit F ; by- 

 means of a fixed lamp a fiducial line could then be marked 

 on the plate, the plate-holder being always put in the same 

 position for this purpose. 



The distance of the source from the centre of the crystal 

 was arranged so as to be exactly equal to the distance of this 

 centre from the photographic plate (about 9 cm.). It is 

 well known that under these conditions no correction for 

 the length of the crystal is necessary in determining the 

 angle of reflexion of the spectral lines, for reflexion of the 

 same wave-length from any point of the crystal always falls 

 at the same point on the plate. The crystal was arranged 

 with the centre of its reflecting face as near as possible 

 opposite the centre of the slit; the plate-holder was adjusted 

 perpendicular to the slit for the zero reading. In making 

 an experiment the crystal was set at a given angle with 

 the central incident ray and the plate-holder rotated through 

 double the angle from the zero position ; the plate was 

 inserted, the fiducial line marked on it, and an exposure 

 of some hours (usually 24) was made, the magnetic field 

 remaining on throughout. The crystals used were rocksalt 

 and heavy spar. The whole apparatus was in a dark room. 



Owing to the finite angle of the beam of 7 rays, and to the 

 length of the crystal, for any given setting of the crystal there 

 are rays striking it at all angles within a certain small range. 

 To enlarge the range and thus obtain more lines on the plate 

 for a single exposure, the crystal was in some cases slowly 

 rotated during the experiment, as in the experiments of 

 M. de Broglie *. The rotation was effected by the following 

 device. Supported by the water in a tall cylindrical vessel 

 was a float, which subsided slowly, owing to the escape of 

 the water, drop by drop, through a capillary tube of suitable 

 size attached to an opening in the bottom of the vessel. The 

 float as it sank rotated the crystal by means of a light 

 horizontal arm, to one end of which it was fastened by 

 means of a thread passing over a pulley ; the other end was 

 attached to the turn-table carrying the crystal. The moving 

 end of the arm was carried without friction by means of 

 wheels on a glass plate, and the motion attained was very 

 uniform and could be adjusted by changing the length of 

 the capillary tube. A rotation of one degree occupied from 

 four to eight hours. 



* M. de Broglie, Journal de Physique, Feb. 1914, p. 101. 



