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SCIENCE PROGRESS 



made by Curtis of all the small nebulae occurring on his plates, 

 and from this count an estimate has been made of the total 

 number of spiral nebulae within the reach of modern large 

 refractors. Estimates had previously been made by Perrine 

 in 1904, who gave a total of 500,000, and by Fath in 191 3, who 

 reduced this figure to 162,000 ; both estimates were based 

 upon counts in a number of selected regions. Curtis very con- 

 siderably increases both of these estimates and places the 

 number at 722,000, with the remark that, since the faintest 

 and smallest members of the class are, in general, discernible 

 only in the central regions of the plate, this figure is an under- 

 estimate, and that the total number accessible with the Crossley 

 reflector with rapid plates and with exposures of from two to 

 three hours may well exceed 1,000,000. A critical discussion 

 of the wide divergence between these three estimates supports 

 the largest of them. The distribution with reference to the 

 Milky Way is of considerable interest, and is summarised as 

 follows : 



Galactic 

 lyatitude. 



+ 45° to + 90° 



- 45° to - 90° 

 ± 30° to ± 45° 



- 30° to + 30° 



The density is greatest in the neighbourhood of the north 

 galactic pole ; there is a less marked concentration near the 

 south galactic plane. A high value of the density persists to at 

 least 60° from the galactic poles, but then shows a remarkable 

 diminution in the neighbourhood of the Milky Way itself. 



Part II contains a study of occulting matter in the spiral 

 nebulae, also by H. D. Curtis. It is well known that certain 

 spiral nebulae, when seen edgewise on, show a dark lane running 

 down the length of the spiral, which is generally explained as 

 being due to absorption of light by the outer layers of the 

 nebulae. Curtis shows that this phenomenon is more common 

 than had been supposed, and excellent reproductions are given 

 of 'J'] Crossley photographs of spiral nebulae which show the 

 dark band. Arguments are brought forward to support the 

 view that the appearance is not due to a phase effect. 



The planetary nebulae, which are very few in number and 

 have a distribution differing widely from that of the spiral 

 nebulae, are considered by Curtis in Part III. A valuable 

 feature of this paper is a series of pictures of all the known 

 planetary nebulae, 78 in number, which are north of 34° south 

 declination. The wide divergence of forms shown by these 

 nebulae, including helical and ellipsoidal formations, rings 

 with and without nuclei, series of concentric shells, etc., makes 



