308 CARNEGIE INSTITUTION OF AVASHINGTON. 



the resuscitated Herschellian hypothesis. Every additional working 

 hypothesis is a gain, and a competitive hypothesis of the origin and 

 dynamics of spiral nebulae should be specially serviceable in stimulating 

 observation and in drawing forth a critical interpretation of results. 

 The working hypothesis elaborated in connection with these studies, 

 based on dynamic encounter incidental to close approach — first put 

 forth in 1901^ and re-stated in greater detail in Year Book No. 3, 

 1904, pp. 208-219 — has heretofore been rather lonesome in its field and 

 should profit by competitive companionship. 



As specifically pointed out in the Year Book just mentioned, the 

 assignment of the origin of spiral nebulae to dynamic encounter is 

 not a necessary part of the planetesimal hypothesis, the chief theme 

 of which is the origin of planets rather than nebulae. In so far, however, 

 as an evolution of planets involves a nebula, an interpretation of the 

 origin of the nebula is necessary to carry the evolutionary process 

 back through an appropriate cycle to what may be regarded as an 

 original or an equilibrium state, the more so because a nebula gives 

 no signs of being in such equilibrium state; its form rather distinctly 

 implies that it is in a transitional state, if not a nascent or juvenile 

 state. If the partial revival of the Herschellian view — hmited to the 

 white nebulae — is to serve as an effective working hypothesis in any other 

 than a formal sense, it needs specific development on its genetic side, as 

 also on its dynamic side, for the spirality of these nebulae seems to imply 

 a specific genesis actuated by distinctive dynamics. It has been a part of 

 the work of the year to endeavor to see how far the doctrine of dynamic 

 encounter by close approach can be associated with the revived Her- 

 schellian view so as to give it appropriate genetic and dynamic qualities. 



As heretofore set forth in these studies, the hypothesis of close 

 approach has been deployed in little more than its planetary relations, 

 but it is obviously applicable to higher orders of organization. If a 

 cluster of stars, of any order, passes near another such cluster, or pene- 

 trates it eccentrically, a spiraloidal effect seems inevitable. It would 

 seem that dynamic encounter might at least serve as an adequate 

 agency for the generation of such obscure spirality as that of our own 

 galactic system, and that of the Magellanic clouds, if indeed they are 

 really spiraloidal at all. It remains an open question, however, 

 whether a star cluster would be thrown by such encounter into an 

 extended, two-armed, sharply defined spiral deployment, such as con- 

 stitutes the most declared type of spiral nebulae. 



The main reason for doubting the competency of dynamic encounter 

 to produce the highly deployed two-armed spirals lies in the apparent 

 absence of a special deploying force, bilaterally controlled. In the 

 planetary application of dynamic encounter set forth in these studies 



lAstrophysicalJournal, xiv, 1, July, 1901, pp. 17-20; Jour. Geol.,ix,5,July-Aug. 1901, pp. 369-393. 



