May 23, 1878] 



NATURE 



89- 



out by means of the accompanying copy of a photo- 

 graph. A small mass of metallic calcium has been 

 placed between the two poles of an electric lamp. By 

 means of the passage of the current this mass of calcium 

 has been raised to a very high temperature, and it has 

 been driven to its ultimate fineness, — it has been driven, 

 in fact, to a state of vapour competent to give out very 

 thick bright lines in the regions marked a, b, c (Fig. 6.) 

 As the vapour is gradually given off from the mass of 

 metallic calcium, it has surrounded the interior part, and 



has gradually cooled as it got aAvay from the action of 

 the electricity. So that here we have an intensely heated 

 mass of calcium vapour in the centre surrounded by a" 

 mass of calcium vapour which is cooling. 



We have, therefore, between us and the central mass 

 a screen of calcium molecules under exactly the same 

 conditions as those we suggested in the screen of mole-' 

 cules giving out 'yellow light lying between us and a 

 distant light source. 



What then are the facts ? Cn the photograph three 



Fig. 6. — Reversal of the light of hot calcium vapour by cool calcium vapour. 



bright broad bands of light are seen in different parts of 

 the spectrum, which represent three of the characteristic 

 bands of the metal calcium. I want especially to call 

 attention to the fact that in the middle of these bands, 

 especially in the one lettered a, there is a fine line of 

 intense blackness. 



That is to say, in that particular region of the spec- 

 trum there has been no light to paint an image of the 

 slit. What has become of that light? The light has 

 been at work, not on the photographic plate, but among 

 the cooler exterior molecules of calcium which have used 

 it up. 



I shall now take it for granted that the great principle, 

 that molecules absorb the light passing through them in the 

 same wave lengths as those which they give out when vi- 

 brating on their own account, has been rendered familiar. 



We must not forget that this statement is only true so 

 long as the molecular combinations are the same, and 

 that we only get this result in the shape of bright lines 

 when we are dealing with the ultimate finenesses of 

 each chemical substance, that is, when we are dealing 

 with each chemical substance in a state of vapour. 



Without going further, then, it is clear that we are 

 now in presence of two causes of coloured light as 

 opposed to white light. There is, so to speak, a cause of 

 efitct and a cause of deiect. We now know of one reason 

 Avhy light may be red : the luminous vibrating substance 

 may only be competent to vibrate at that particular rate 

 which gives us the sensation of red hght. But this is 

 not the only reason why light may be red. If we 

 assume a screen so constituted that all the light proceed- 

 ing from a white light source, except the red, should be 

 absorbed by the screen, we have there a condition in 

 which the sensation of red would again be produced. In 

 this case it will not be the <?/"fect of red vibrations alone 

 in the light source, but by virtue of the rt'rfect of all the 

 others which we have assumed to be absorbed by the 

 screen. 



In both cases we arrive at 



VIBGYOK 



in the first case because the only light given us is 



in the second because the screen vibrates only in 



w a © © Y © 



and therefore only absorbs these colours. 



Red fire in a pyrotechnic display is an example of the 

 first case. The setting or rising sim is an example of the 

 second. 



The expenditure of a very small amount of money 

 and time will enable any one to become acquainted with 

 many of these phenomena. The best spectroscope after 

 all, perhaps, ease of manipulation being taken iiito' 

 account, is a prism held close to the eye, and a fine slit, 

 say one-twentieth of an inch broad and two inches long, 

 carefully cut out of a piece of tin-foil, gummed on a 

 plate of glass. When this slit, say a foot off, is observed 

 with the refracting angle of the prism parallel to its 

 length, a very brilliant spectrum of a candle just in. 

 front of the slit is obtained, even though it be 

 wanting in definition. This latter can of course be 

 improved if a narrower slit be employed : for in spectra, 

 all impurity comes from overlapping of images, and the- 

 operations of Nature are so fine that it seems as if a pure 

 colour, such as a pure blue or a pure red, will for ever 

 remain an abstraction ; for, however great the disper- 

 sion, the adjacent wave-lengths Avill remain commingled, 

 and commingled wave-lengths define a compound colour. 



Instead of reducing the width- of the slit, if it is not 

 connected with the prism by means of a tube, as it rhay 

 conveniently be, the slit can be removed further frofn 

 the prism. In this way we get apparently a narrower 

 slit without any reduction in the quantity of light which 

 passes through it to the eye. A gas flame or a candle 

 placed in front of this slit is all that is necessary to 

 produce a continuous spectrum. 



J. Norman Lockyer 



,J CLIFFORUS DYNAMIC 



Eiemenis of Dynamic. Part I. Kinematic. By W. K. 

 Clifford. (London: Macmillan and Co., 1878.) 



THOUGH this preliminary volume contains only a 

 small instalment of the subject, the mode of treat- 

 ment to be adopted by Prof. Clifford is made quite 

 obvious. It is a sign of these times of real advance, and 



