344 



KNOWLEDGE. 



September, 1911. 



of these objects was published by Dr. Dreyer after 

 the death of Sir John HerscheL and additional lists 

 containing objects discovered by photograpliy and 

 in other ways have, perhaps, brought up the number 

 to double that contained in Herschel"s catalogue. 

 The most remarkable discovery in stellar, or perhaps 

 \\e had better say sidereal, astronomy during the last 

 half-century is that of the existence of vast numbers 

 of spiral or " whirlpool "* nebulae. Only a \'ery few 

 of these objects had been previously known, the 

 first having been detected by Lord Kosse, about 

 1840, with the help of his great reflector, which 

 showed the spiral form of the great nebula in Canum 

 Venaticorum and others previously regarded as ring 

 formed or annular. 



Sir \\'illiam Herschel divided these objects into — 

 (1) Clusters of stars in which the stars are easily 

 distinguishable : these are again subdivided into 

 ^/o6/(/t7r and irregularclusters. (2) Resolvable nebulae 

 or such as excite the suspicion that the_\- consist 

 of stars, and which any increase of optical power 

 may be expected to resolve into distinct stars. 

 (3) Nebulae, properly so called, in which there is no 

 appearance whatever of resolvabilit}' ; these again 

 were subdivided into subordinate classes according to 

 their brightness and size, and so on. (4) Planetary 

 Nebulae, (5) Stellar Nebulae, and (6) Nebulous 

 stars. 



Long previousK' to Herschel's dux. Hallex' and 

 other earlier discoverers of nebulae had followed the 

 speculations of some philosophical w ritcrs in postu- 

 lating the existence of a primitive elementary form 

 of luminous sidereal matter, (Halle\-. Philosophical 

 Transactions. \'ol. X.\L\. page J90. ct sci/.K but 

 it was mainl\- as a result of his extensive observa- 

 tions that Sir William Herschel himself was led to 

 consider the consequences of a gradual subsidence 

 and condensation of such matter into more or less 

 regular spherical or spheroidal forms denser tow ards 

 the centre than at the circumference. Laplace's 

 well-known nebular hypothesis, worked up hv him 

 from the previous suggestions of Kant. \\'right. 

 Swedenborg, and others, also brought into [iromi- 

 nence this idea of " nebulous matter " nt a nature 

 distinct from ordinary solid or fluid substance such 

 as we are familiar with on our planet. 



The application of the spectroscope to Astronom\-. 

 about the year 1860, threw a flood of light upon 

 subjects which had been thought to be bevond the 

 powder of the mind of man to discover, such as the 

 chemical nature and ph}-sical condition of the 

 heavenh' bodies and enabled us to draw the real 

 distinction between true nebulae and ap[)arent 

 nebulae, or remote and condensed clusters. Newton 

 in his " Optics " relates that he once decided to "trx 

 the celebrated experiment of the colours." Admitting 

 sunlight through a small opening into a darkened 

 room, if allowed to fall normalh' on a white screen it 

 produces a round white spot, which is an image of the 

 sun. If now we introduce a prism or triangular piece 

 of glass, with its edge downwards, in the path of the 

 beam, the latter will be deflected upwards, and the 



image on the screen will be no longer white 

 but coloured, a man\- hued band, showing the 

 " colours of the rainbow." The i)rism. in fact, will 

 have effected an analysis of the light. White light 

 (sunlight) is composed of a mixture of different 

 colours, and these on passing through the prism are 

 deviated to a greater or less degree, the violet ra}-s 

 most refracted and the red least so. By employing 

 a \'er\- narrow slit cUid a number of prisms (or a 

 diftVaction gratmg) a pure spectrum or coloured band 

 of considerable length, in which the colours are well 

 separated, is obtained. By such means Wollaston, 

 and after him Frauenhofer, showed that the 

 ■■ spectrum " gi\^en by sunlight is not quite con- 

 tinuous, but crossed b\' a number of fine dark lines 

 or \-acant spaces, if the slit is illuminated by a 

 gas flame or lamp such lines are not seen, but a 

 continuous spectrum is obtained, giving the colours 

 red, orange, vellow, green, blue (indigo) and x'iolet, 

 the so-called seven colours of the rainbow. Thus 

 the dark lines are not characteristic of light in 

 general, but only of sunlight. By special means 

 it was shown that the brighter stars, too, gave spectra 

 containing svstems of dark lines somewhat similar 

 to those given bv sunlight, but with differences for 

 different stars. 



Passing over various stages in which the meaning 

 of these lines, known universalh" as the Erauenhofer 

 lines, was gradually unfolded, we come to the work 

 of Kirchhoff and Bunsen. They showed that a con- 

 tinuous spectrum is given by every incandescent body 

 whose molecules are so entangled as to be unable to 

 \ibrate freely ; in other words, by solids, liquids and 

 gases under high pressure. A gaseous substance, 

 under low pressure, gives a discontinuous spectrum, 

 containing only a few briglnt lines, and these lines 

 are characteristic for each elementary substance, and 

 also for some compounds. \ gaseous substance, 

 when cooler, absorbs from white light passing 

 through it, precisely those rays which itself emits 

 when hot. Thus the spectrum of white light, 

 after passing through sodium \apour. exhibits 

 two distinctive dark lines in the yellow, whilst 

 inciuulescent sodium emits a yellow light, which, 

 when examined by the spectroscope, is found 

 to be of just that refrangibilit>-. Thus it was 

 discovered that somewhere between the Sun's 

 luminous surface and the earth, there are a number 

 of vajiours of well-known elements, which form 

 the ■■ dark " Frauenhofer lines. Applying the 

 spectroscope to the nebulae and clusters, a matter of 

 some difficulty owing to the faintness of their light, 

 the late Sir William Huggins. aptly named by 

 Proctor the " Herschel of Spectroscopy," found that 

 man\- of the former bodies gave spectra consisting 

 of five or six bright lines, indicating the presence of 

 hydrogen, magnesium, and nebulium (?) under low- 

 pressure, in a state of great rarefaction. 



.Some, such as the great nebula of .Andromeda, 

 give a continuous spectrum unmarked by lines or 

 bands whether bright or dark. It is possible these 

 ma\' consist of more condensed gases, but this is by 



