February i6, 1899] 



NATURE 



371 



A SIMPLE SPECTROSCOPE AND ITS 

 TEACHINGS. 



I. 



SPECTRUM analysis is now becoming so far-reach- 

 ing, especially in inquiries having to do with the 

 conditions of the various celestial bodies, that there are 

 many who are anxious to know something of its teachings. 

 To some of these, however, the terms used by men of 

 science, a very necessary shorthand, are unfamiliar and 

 appear hard to understand, because the opportunity of 

 seeing the things they are intended to define, and which 

 they generally do define in most admirable fashion, has 

 never presented itself. I propose, therefore, to attempt to 

 show that there is nothing recondite about these terms ; 

 that it is possible without any e.xpensive apparatus for 

 every one, who will take a little trouble, to observe the 

 phenomena for himself, after which the meanings of the 

 terms employed will present no difficulty whatever. 



One key to the hieroglyphics, the light story, which is 

 hidden in every ray of light, is supplied to us by the rainbow. 

 It teaches us that the white light with which nature bounti- 

 fully supplies us in the sun's rays, is composed of rays of 

 different kinds or of different colours ; and it is common 

 knowledge that there is an almost perfect analogy between 

 these coloured lights and sounds of different pitches. 



The blue of the rainbow may be likened to the higher 

 notes of the key-board of a piano, and the red of the 

 rainbow, on the other hand, may be likened to the longer 

 sound waves, which produce the lower 

 notes ; and as we are able in the language 

 of music to define each particular note, 

 such as B flat and C. sharp, and so on, 

 so light-waves are defined by their colours 

 or wave-lengths. 



What nature accomplishes by a rain-drop, we can do 

 with a prism or a grating. A prism is a piece of glass 

 or other transparent material through which the light 

 is bent out of its course or refracted in the process. 

 A grating is a collection of wires, or scratches on glass 

 or metal ; equidistant, very near together and all parallel. 

 When light passes through or is reflected by such a 

 system it is said to be diffracted, and one result that we 

 are concerned in, is very similar to that of passing light 

 through a prism. 



It is rapidly becoming a familiar fact to many that 

 when a ray of white light is refracted by a prism, or i 

 diffracted by a grating, a band of colour similar to a rain- l 

 bow is produced, and that this effect follows because ; 

 white light is built up of lights of every colour, each colour i 

 having its own special length of wave and degree of j 

 refrangibility. Our rambow band is called a spectrum. ! 



.Such a glass prism or grating is the fundamental part 

 of the instrument called the spectroscope, and the most 

 complicated spectroscope which we can imagine, simply 

 utilises the part which the prism or grating plays in 

 breaking up a beam of white light into its constituent 

 parts from the red to the violet. Between these colours 

 we get that string of orange, yellow, green and blue, which 

 we are familiar with in the rainbow. 



For si.xpence any of us may make for ourselves an 

 instrument which will serxe many of the purposes of 

 demonstrating some of the marvellously fertile fields of 

 knowledge which have been recently opened up to us. 

 From an optician we can get a small prism for bd.\ get a 

 piece of wood from 20 to 10 inches long (the distance of 

 distinct vision), i inch broad and \ an inch thick. On 

 one end glue a cork 2 inches high ; at the other end fasten, 

 by melting the bottom, a stump of a wa.\ candle of such 

 a height that the dark cone above the wick is level with 

 the top of the cork. Then glue the prism on the cork, 

 so that by looking sideways through the prism the 

 coloured image, or spectrum, of the flame of the candle 

 placed at the other end of the piece of wood can be seen. 

 NO. 1529, VOL. 59] 



We get a band ot colour, a spectrum of the candle 

 flame, built up of an infinite number of images of tlie 

 flame produced by the light rays of every colour. But, 

 so far, the spectrum is impure because the images over- 

 lap. We can get rid of this defect by replacing the 

 candle by a needle. 



If we now allow the needle to reflect the light of the 

 candle flame, taking care that the direct light from the 

 candle does not fall upon the face of the prism, we then 

 get a much purer band of colour, because now we have 

 an innumerable multitude of images of the thin needle, 

 instead of the broad flame, close together. The needle is 

 the equivalent of the slit of the more complicated spectro- 

 scopes used in laboratories. 



We can vary this experiment by gumming two pieces of 

 tin-foil with two perfectly straight edges on a piece of 

 glass so that the straight edges are parallel and very near 

 together. In this way we have a slit ; this should be 

 fi.xed close to the candle and between it and the prism. 



Now the light of the candle is white, and the preceding 

 experiment tells us that such light gives us a band con- 

 taining all the colours without any breaks or gaps. We 

 have what is called a continuous spectrum. 



The Continuous Spectrum. 

 If we burn a piece of paper, or a match, or ordinary coal- 

 gas, we get a white light identical to that given us by the 

 candle ; solids which do not liquefy when made white- 

 hot, and liquids which do not volatilise under the same 



Arrangement of candle, pii; 



condition ; and some dense gases when heated, do the 

 same. 



This effect is produced because there is light of every 

 wave-length to produce an image of the needle (or the 

 slit) ; these images blend together continuously from one 

 end of the spectrum to the other. 



Let us then consider this fact established, namely, that 

 solid or liquid bodies and dense gases, when heated to a 

 vivid incandescence, give a continuous spectrum. Under 

 these circumstances the light to the eye, without the 

 spectroscope, will be white, like that of the candle. 



T/ie Length of the Continuous Spectrum varies luith 

 Temperature. 



If we put a poker in a fire, it becomes red-hot ; if we 

 heat a platinum wire by passing a feeble current of 

 electricity along it, it becorries red-hot like the poker. 



In both cases examination by means of the prism 

 shows that the red end only of the spectrum is visible. 

 But if the poker or wire be gradually heated more 

 strongly, the yellow, green, and blue rays will successively 

 appear. Finally, when a brilliant white heat has been 

 attained, the whole of the colours of the spectrum will 

 be present. 



Hence we learn that if the degree of incandescence 

 be not high, the light will only be red. But, so far as the 

 spectrum goes — and it will expand towards the violet, as 

 the incandescence increases, as before stated — it will be 

 continuous. 



The red condition comes from the absence of blue 



