218 



DISCOVERY 



contains at birth 35,100 ova, but that these are reduced 

 by degeneration to 11,000 after twenty-three days, and 

 to 6,000 by the sixty-third day. That nutrition must 

 play an important part in regulating the proportion 

 of eggs which survive and eventually become mature 

 is a conclusion which is based on observation and not 

 merely on inference. This is a matter which wiU be 

 considered in a further instalment of this article, 

 dealing with some of the factors which control fecundity 

 in the domestic animals. 



Among the Stars — 

 A Monthly Commentary 



Discovery of 850 New Nebulae 



In Harvard College Observatory Bulletin, No. 784, Dr. 

 Harlow Shapley announces his discovery of 850 new nebulae 

 on a plate exposed on September 19 of last year, with 

 an exposure of six liours. Dr. Shapley announces that 

 these are " distinct nebulas," the brighter almost without 

 exception being elongated or spiral, while the fainter are 

 mainly globular in shape. " At the eighteenth magnitude, 

 on many parts of the plate, nebute are more numerous 

 than the stars." 



A New Theory of the Spiral Nebulae 



Perhaps the most difficult problem in the whole realm 

 of astronomy at the present moment is that of the nature 

 and status of the spiral nebulae. During the last decade, 

 two main theories have been supported by strong observa- 

 tional evidence. These are (i) the older, or nebular, 

 theory, the exponents of which regard the spirals as true 

 nebulae, comparatively near at hand as celestial distances 

 go; and (2) the "island-universe" theory, whose sup- 

 porters maintain that the spirals are galactic systems, 

 situated at immense distances from our own stellar 

 universe. Within the last year or two, the tide of opinion 

 has begun to flow somewhat strongly against the island- 

 universe theory, without giving definite support to its 

 rival. At a recent meeting of the Royal Astronomical 

 Society, Professor F. X. Lindemann outlined a novel 

 hypothesis which is embodied in the society's Monthlv 

 Notices, April 1923, pp. 354-9. According to this theory 

 the spirals consist of dust-particles, repelled from the stars 

 by radiation pressure. " There must be a continuous 

 stream of fine dust leaving the neighbourhood of the stars 

 and moving towards regions of low radiation density." 

 On this hypothesis, the very high velocities of the spirals 

 are accounted for as a consequence of light-pressure, 

 which may rise to eight times the gravitational attraction 

 in the case of the sun, while " for stars radiating more 

 per unit, it may be considerably greater." Dr. Lindemann 

 further suggests that the light of the spirals is not inherent, 

 but due to reflection from the light of the stellar system as 

 a whole. His explanation of the appearance of novse 



in the spirals — that these outbursts are due to encounters 

 between spiral nebulae and " comet-like clouds of stones 

 or sand " — is, on his own admission, " somewhat artificial." 

 The new hypothesis, which is not out of harmony with 

 certain speculations of Dr. See and a tentative theory 

 outlined by Dr. Shapley, is certainly worthy of serious 

 consideration. 



A Stellar Mystery 



The dailv press reported early in May the discovery 

 of a new star of between the fifth and sixth magnitude in 

 the constellation Cygnus by Dr. Thomas D. Anderson, 

 the Scottish astronomer, at Thurston Mains, Innerwick. 

 Dr. Anderson detected Nova Aurigas in 1892, and Nova 

 Persei in 1901. .\ search at the great observatories, how- 

 ever, did not confirm the discovery. Professor Frost 

 reported that visual and photographic observations made 

 at the Yerkes Observatory on May 12 failed to show any 



AN EX.\MPLE OF A SPIR.\Ij NEBULA. 



The great spiral uebula, M51, in Canes Veuatici. 



(Photo by Dr. Max Wolf.) 



nova near to the position assigned by Dr. .\nderson, while 

 Dr. Shapley reported that on several Harvard photographs 

 showing stars fainter than the eleventh magnitude no 

 nova could be traced. 



