138 
The N.Z. Journal of Science and Technology. [June 
1 : 2 : 4 : 8. Let us suppose that their velocity, which is approximately 
the same for all, is sufficient for circular motion. Then, when A has 
completed one revolution, B, C, D will have reached the positions 
B', O', D'. Similarly, the stars E, F, G, H will have reached the positions 
E', F', G', H', and the stars that were in line will be spread along two 
spiral branches. 
It is clear that whilst the encounter is in progress the line of relative 
streaming through the nucleus is at right angles to the line DH’. 
Fig. 2 shows the influence of time on the spiral form. The dotted curve 
represents the same mateiial as the continuous line, but at the epoch in which 
twice the interval has elapsed since the encounter. Thus the number of 
complete turns in a single branch gives a clue to the age of the spiral. 
Fig. 3 shows the form of the arms in the case in which the central 
condensation is so great that the mass outside the nucleus is negligible 
in comparison with that within it. The orbits of the stars range from 
ellipses to hyperbolae, but the spiral arms pass through approximately 
the same succession of forms. 
Thus the spiral nebulae appear to be the natural result of the inter¬ 
penetration of cosmic systems. The stars composing them do not appear 
