1903.] on the New Star in Gemini. 381 



that all round Nova Persei there is one of those dark nebulas of which 

 we have spoken, and that the brilliant illumination of the star in the 

 first week is successively lighting up for our inspection the parts of 

 this nebula further and further from the centre. 



There are some difiiculties about this explanation which I do not 

 wish to under-estimate, though I think they may be smoothed away. 

 Some astronomers, for instance, feel it to be a difficulty that in the 

 actual case of Nova Persei the expansion is at many points decidedly 

 not outwards from the centre. But a little further consideration of 

 our analogy will show that we must not expect this symmetrical 

 expansion. We have previously supposed the solar system to be 

 viewed from above, but let us now shift our imaginary view point to 

 a position in the plane of the orbits. It is easy to see that Saturu 

 may now appear either (1) on the side of the earth remote from th( 

 8un, or (2) between the Sun and the Earth. In the former case the 

 illumination which reaches the Earth before Saturn will appear to 

 be travelling in the normal direction outwards, but in the latter case 

 it will actually appear to travel inwards, exactly contrary to expecta- 

 tion. And it is easy to see that, by choosing our view point, we could 

 obtain any anomaly in the perspective view, so that such anomalies 

 in the case of the illumination of a vast irregular nebula need not 

 trouble us. 



On the other hand this explanation of the phenomena has received 

 during the last few weeks a remarkable confirmation. If the light 

 we are now receiving form the nebula is the light of the original 

 flare of the star, we ought to be able to identify it by its colour, or 

 by its spectrum which is a glorified name for colour. If, for instance. 

 Nova Persei had flared up with a ^erce red light which afterwards 

 turned to green, and the nebula were to shine with a green light, we 

 could not entertain the view that the nebula owed its illumination 

 to the original red flare : on the other hand, a red light from the 

 nebula would so far confirm that view of its origin. Now substitut- 

 ing for the terms red and green two varieties of spectrum, this has 

 been found to be the case : the spectrum of the nebula agrees with 

 the spectrum of the star at the outburst, and not with its subsequent 

 spectrum. 



This result is easily stated, but its attainment represents a truly 

 marvellous achievement in astronomical photography. The light of 

 the nebula round Nova Persei is so faint that an exposure of some 

 hours is necessary to photograph it at all, even with a powerful 

 telescope which would take a picture of the moon in the hundredth 

 of a second. To get the spectrum, each point of light must be 

 spread out into a line, which dilutes the already faint illumination 

 very considerably, and no one could be blamed for regarding such an 

 enterprise as hopeless. But at the Lick Observatory they have 

 learnt by a long series of successes, to despair of nothing. With the 

 help of his brother astronomers, Mr. Perrine devised a special instru- 

 ment for the undertaking which would give him the best possible 



