58 SCIENCE PROGRESS 



puscles are always to be found there, the result being that 

 the behaviour of the positively charged particle is complicated 

 in the following ways : 



It may pick up a single negative charge and becoming 

 neutral may pass the fields unaffected and strike the plate at 

 the origin O, the " undeflected spot." 



It may pick up yet another negative charge before emerg- 

 ing from the canal-ray tube and by retaining this throughout the 

 fields may become a Negative Primary Ray and give rise to 

 a parabola similar in all respects to the positive ones but in 

 the opposite quadrant, as shown in fig. 5 by dotted lines. 



It may be changed from a neutral to a charged particle of 

 either sign or vice versa during its passage through the fields, 

 thereby giving rise to rays which do not strictly obey the 

 fundamental equations, as the values of X and H which affect 

 them will not be constant but will depend on the position of 

 their origin or destruction. These are called " Secondary Rays." 

 The effect of these rays on the photograph or screen is ex- 

 ceedingly complex ; indeed in the earlier experiments they 

 completely overshadowed the genuine primary rays, so that 

 it was only by designing apparatus in which the [pressure in 

 the camera could be kept low that the primary rays could be 

 seen distinctly. Even with the apparatus in its present state, 

 it is impossible to eliminate them entirely, especially when gases 

 such as hydrogen or helium are present which are not com- 

 pletely absorbed by the cooled charcoal. Owing to the presence 

 of secondary rays, the greatest care must be taken in interpre- 

 ting the photographs, as the secondary rays may give parabolas 

 which under certain conditions are quite indistinguishable from 

 the true primary parabolas. Fortunately the relative positions 

 of these false curves are usually changed when the photograph 

 is repeated under slightly different experimental conditions. It 

 is then possible to detect them, as no such change in the 

 relative positions of the true primary parabolas is ever noticeable. 

 The object in maintaining the lowest available pressure in the 

 camera is to eliminate secondary rays as far as possible. 



It will now be well to consider a few of the actual results 

 in detail. The accompanying plates are reproductions from the 

 original negatives and illustrate several typical cases. Plate I 

 was obtained with nitrogen (made from air) in the tube, the 



