490 



Mr. C Gr. Darwin on the Collisions of 



approaches from a fixed direction, so that the position of the 

 initial line of a is the only variable. Take a plane throngh 

 H perpendicular to the direction of this line, and choose 

 some point of «, which may be called its centre. The line 

 of approach may then be completely described by the point 

 where the centre of a would cut the plane, if there were na 

 deflexion. So, given this point, the orbit is determinate and 

 with it 6, the angle of projection of H. Corresponding to* 

 every point of the plane there is a value for 0, and we can 

 thus mark out on it curves of constant 6. We shall call the 

 figure so made a projection diagram ; it will be on a definite 

 scale, of the order of 10~ 13 cm. in the actual problems we 

 are going to consider. Fig. 1 is a hypothetical example^ 



Ffc.l. 



Hypothetical Collision Diagram. 



Each number in the figure indicates the number of degrees of arc of the 

 angle of projection in the corresponding collision. 



Now we define a quantity P as the area on the diagram for 

 which the angle of projection is less than 6. If a and fl can 

 have variable orientations, P is the averaged area in all the 

 projection diagrams that may occur. Then in an experiment 

 the number of H-particles projected at angles less than 6 

 will be equal to the product of P by a number of known 

 factors, the latter depending on considerations of probability. 

 Therefore, all that experiment can do is to determine the 



