NA TURE 



[February i6, 1899 



light ; the white condition comes from the gradual 

 addition of blue as the temperature increases. 



One of the laws formulated by Kirchhoff in the infancy 

 of spectroscopic inquiry has to do with the kind of 

 radiation given out by bodies at different temperatures. 

 The law affirms that the hotter a mass of matter is the 

 further its spectrum extends into the ultra-violet. 



Gaslight is redder than the light of an incandescent 

 lamp because the latter is hotter. The carbons in a so- 

 called arc-lamp give out a bluish-white light because 

 they are hotter still. 



By similar reasoning from experiment we are bound 

 to consider the bluish-white stars, the white stars, the 

 yellow, red and blood-red stars to indicate a decreasing 

 order of temperature.' 



We shall not go far wrong in supposing that the star 

 with the most intense continuous radiation in the ultra- 

 violet is the hottest, independently of absorbing condi- 

 tions, which, in the absence of evidence to the contrary, 

 we must assume to follow the same law in all. 



An inquiry into the facts placed at our disposal by 

 stellar photographs, shows that there is a considerable 

 variation in the distance to which the radiation extends 

 in the ultra-violet, and that the stars can be arranged in 

 order of temperature on this basis. 



Judged by this criterion alone, some of the hottest 

 stars so far observed are y Orionis, f Orionis, a \'irginis, 

 y Pegasi, r; Ursae Majoris, and X Tauri. Of stars of 

 lower, but not much lower, temperature than the above, 

 may be named Rigel, f Tauri, a Andromedas, ^ Persei, 

 a Pegasi, and /3 Tauri. 



In this way spectrum analysis helps us with regard to 

 temperatures^ both on the earth and in the heavens. 



Discontinuous Spectra zvith Bright Lines. 



Let us next pass from a solid which retains its incan- 

 descence like platinum wire without melting, or a liquid 

 which retains its incandescence without volatilising, like 

 molten iron, and see what happens. We have found that 

 when the light entering the slit consists of every colour 

 and every tone, we have a continuous band of colour. 

 If there be any defect in the light we must have a dis- 

 continuous one, for the reason that an image of the slit 

 cannot be produced in any particular part of the spectrum 

 if there be no light of that particular colour to produce 

 it when we deal with coloured flames or vapours or 

 gases rendered incandescent by electricity. 



There are many artificial flames which are coloured, 

 and if their light be analysed in the same way as the 

 light of the candle, a perfectly new set of phenomena 

 present themselves. 



Let us again make use of our improvised spectroscope, 

 and allow the needle to be illuminated by the flame of a 

 spirit lamp into which salt is gradually allowed to fall ; 

 we see at once why the flame is orange-coloured. It 

 contains no red, yellow, green, blue, or violet rays, so 

 that we should not represent the spectrum by 



w a [13 (c^i V © [^ 



as in the case of the candle, but simply by 



¥ 



We see one image of the needle coloured in orange. 

 We have passed from the spectrum of polychromatic 



. idea wiih re- 

 been held Ity 

 lagined 



I On tlii^ point I wrote as follow-i in iS^^a ; " .4 

 gard 10 the indications of the temperature of ihc stars h 

 thusc who have not con^tidered the matter specially. It h; 



itself ^uflici< 



Cornu, howc 



with an ordinary spark without jar. Hence the hiRh temperature of such 



star at Sirius i.snot indicated by the fact that its spectrum shows the whole 



Rerie* of hydrogen Xm^s, but fy the fact that tha 



radiation far in the 



t evidence of a very high temperature. The experiments of 

 :r, have shown that the complete scries u( lines can be seen 



but by the 

 a-viofetr 



NO. 1529, VOL. 59] 



ccntin 



to that of monochromatic light — from white light to 

 coloured light— from light of all wave-lengths to light of 

 one w-ave-length ; from an infinite number of slit images 

 giving a continuous band of every colour, to one image 

 of the slit produced by light of one refrangibility, the 

 colour of the image depending upon the refrangibility. 

 What we shall see in passing from the spectrum of the 

 candle to that of sodium vapour in the spirit lamp is 

 shown in the accompanying woodcut. 



That we are truly dealing with an image of the needle 

 (or a slit) can l)e proved by using a slit of any shape. 

 This can be shown by slightly altering our needle ex- 



periment. Take a piece of glass and a piece of tin-foil 

 li inches square, cut out of the centre of the tin-foil 

 a disc slightly larger than a threepenny-piece, and gum 

 the remainder on the glass. In the centre, where the 

 disc has been cut away, gum a threepenny-piece. The 

 interval between the threepenny piece and the tin-foil con- 

 stitutes a circular slit. Let it replace the needle, and 

 examine the flame of the spirit lamp charged with salt 

 through it with the prism as before. 



It will readily be grasped, from what has been stated, 

 that in the case of coloured flames, the light passing 

 through the spectroscope being only red, or yellow, or 



spectroscope 



./, slit ; t /, 



scale illumin- 



flames to be coinp.-ired ; /, observinR telesc 



ated by h ,nnd reflected by the second surface of the prism into 



green, as the case may be, will go to build up an image 

 of the slit in the appropriate part of the spectrum, and 

 that the image thus built up will take the form of a line 

 or circle, accortling to the slit we use. 



Many chemical substances, salts or various metals, 

 become luminous by inserting them into llames, as we 

 have treated tomniDn salt (chloride of sodium). With 

 each metal tin- colour imparted to the flame is different. 

 The resulting spectrum is called a discontinuous spectrum, 

 because it is only here and there that images of the slit 

 are produced ; because some coloured rays, and not all, 

 are present. 



