54 



KNOWLEDGE. 



[February 1, 1897. 



No. 8, spiral nebula in Ursa Major, E.A. 10b. 12m., 

 Decl. 41" 57' nortb. Exposure, -lb. on tbe 14tb April, 

 1893. Scale, one millimetre equals twelve seconds of 

 arc. 



No, 4, spiral nebula in Canes Venatici, E.A. 13h. 25m., 

 Decl. -il 45' nortb. Exposure, 2b. 55m. on tbe 15tb 

 May, 1S9G. Scale, one millimetre equals twelve seconds 

 of arc. 



It will be seen on examination of tbese pbotograpbs 

 that tbe nebiilous matter in tbe spirals is broken up into 

 stars and starlike loci, and tbese vary in brightness between 

 tbe light of stars of about the seventeenth magnitude 

 and that of about tbe thirteenth magnitude. The bright 

 stars are as well defined round their margins as are any 

 other stars in the sky, whilst the fainter ones are nebulous 

 and not well defined round their margins ; and those that 

 differ but little in brightness from the nebulosity in which 

 they are immersed have generally margins more or less 

 undefined. 



Every spiral nebula that I have photographed has a 

 stellar nucleus surrounded by dense nebulosity in its 

 centre of revolution, and around that centre tbe nebulous 

 convolutions and the stars involved in them are sym- 

 metrically arranged. The records of these features are 

 now so numerous and accordant that they cannot be 

 attributed to accidental or fortuitous causes. They must 

 be referred to the action of known physical causes of 

 which we have ample evidence. 



Up to this pomt we have directed attention to the 

 photographic records, which prove with absolute certainty 

 — for they are unafl'ected by personal bias or error — the 

 existence, the forms, and the structures of these nebul;^. 

 We can also measure the phases, tbe structural details, 

 the diameters, and the relative distances apart of the stars 

 involved in them ; but we cannot determine their distances 

 from tbe earth for the reason that no measurable parallax 

 is presented by them. That their distances from the 

 earth are very great is also proved by tbe fact that I have 

 not been able to discover any obvious change to have 

 taken place, during intervals of from four to seven years, i 

 in the stars or in the nebulosity depicted upon those 

 photographs which I have been enabled to compare with 

 one another. It is true that the comparisons referred to 

 are not reliable for distances of less than about three 

 seconds of arc by measurements on the negatives. 



We shall now turn a little aside from the solid path of 

 positive evidence, to make some inferences from it in 

 answer to the questions : Of what materials are the spiral 

 nebulae composed ? Whence the source of the vortical 

 motion which evidently has produced their forms '? We 

 need not proceed far into the domain of speculation to 

 seek an answer, for we have the evidence of Sir William 

 Herschel, of Sir .John Herschel, of Lord Eosse, and 

 since their time tbe evidence of many other competent 

 observers who possessed large optical appliances, by which 

 they discovered that in many regions of tbe sky extensive 

 clouds of nebulous matter exist. Sir William Herschel 

 alone has recorded the positions of fifty-two such regions, 

 and within the past ten years photographs have been 

 taken which confirmed the truth of those eye observations, 

 as well as placed before our eyes accurate structural 

 delineations and definitions of the boundaries of some of 

 those nebulous areas. 



The material in space out of which symmetrical nebulae 

 may be condensed has been proved to be both extensive 

 and widely spread ; but whence the vortical force by which 

 the spiral nebulre have been formed ? 



Considerable evidence from the past is available by 

 which we feel assured that collisions have taken place 



between bodies in space — Nova Auriga; and Xova Andro- 

 meda are recent examples. We know also that every star, 

 planet, nebula, comet, or other object discovered in space 

 is in rapid motion of translation, and that the motions are — 

 with the exception of those stars physically connected — in 

 diverse directions and apparently not controlled by a 

 common central governing force. 



In the case of several of the nebulfp the photogi-apbs 

 show them to be bodies of a lenticular form, because they 

 are viewed edgewise, and are either spiral, elUpsoidal, or 

 circular when \iewed full-face. They are also presented 

 to us at all angles between these extremes, and of course 

 every angle of view presents them with differing outlines. 



There are also nebubi- which have not yet assumed a 

 regular or a geometrical figure : of such is the great nebula 

 in Orion, and tbe Crah nebula M. 1 Tauri. These are 

 probably nebulosities in a less advanced stage of develop- 

 ment than those which have geometrical outlines, and 

 there can be no reasonable doubt that there exists in 

 various parts of space nebulosities which may be classified 

 under tbe divisions following : — 



1. Vast areas of cloudlike matter; gaseous, and 

 probably of discrete solid particles intermixed. 



2. Smaller areas of matter undergoing the process ot 

 condensation and segregation into more regular forms. 



3. Spiral nebula' in various stages of condensation and 

 of aggregation. 



4. Elliptic nebulfe. 



5. Globular nebulse. 



In the three classes last named there is clear evidence, 

 observable on every photograph which has been taken, 

 that condensations into stars, or into starlike forms, of the 

 nebulous matter is now taking place within them. 



Prof. Norman Lockyer, in bis " Meteoritic Hypothesis,'' 

 points out that collisions of meteorites in a swarm would 

 produce luminous nebulosity ; so also would collisions 

 between swarms of meteorites produce all the conditions 

 requisite to account f(ir the vortical motions and the dis- 

 tributions of the nebulosity in the spiral nebulfe ; the 

 starlike aggregations of the nebulous and meteoric matter 

 subsequently taking place, as the photographs show, in the 

 convolutions. 



Spiral nebulip might also be the result of collisions 

 between globular bodies such as the earth. But we will 

 now proceed to point out the apparent relationship between 

 the curves and Imes of stars shown upon the four photo- 

 graphs which accompany the first part of this communica- 

 tion, with the curves of stars which are immersed in the 

 convolutions of the spiral nebulfe as shown on the accom- 

 panying photographs. The subject is not one for argument, 

 but for observation and correlation of tbe photographs ; 

 and I will assume that tbe similarity of the curves of stars 

 respectively upon them is obvious to sight. A little further 

 consideration will also make it clear to us that if we could 

 view these nebube edgewise, the stars involved in the con- 

 volutions would appear as approximately straight lines ; 

 and when the whole of the nebulosity has been absorbed 

 by tbe stars, they will in all respects resemble and emit 

 light Hke the finished stars that we see by the photographs 

 actually existing in space. 



If tbe visible evidence which is now before us is here 

 correctly interpreted, we must accept as a reasonable ground 

 for belief that stellar systems are now in process of evolu- 

 tion out of nebulous and probably meteoric matter ; and 

 that it is only a question of time when changes in the 

 structures, and in the several phases of the nebulse and of 

 the stars, will be made evident on comparison of photo- 

 graphs which have been taken with long intervals of years 

 between them. 



