Nature . periment. Take a piece of glass and a piece of tin-foilli inches square, cut out of the centre of the tin-foila disc slightly larger than a threepenny-piece, and gumthe remainder on the glass. In the centre, where thedisc has been cut away, gum a threepenny-piece. Theinterval between the threepenny piece and the tin-foil con-stitutes a circular slit. Let it replace the needle, andexamine the flame of the spirit lamp charged with saltthrough 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 passingthrough t
Nature . periment. Take a piece of glass and a piece of tin-foilli inches square, cut out of the centre of the tin-foila disc slightly larger than a threepenny-piece, and gumthe remainder on the glass. In the centre, where thedisc has been cut away, gum a threepenny-piece. Theinterval between the threepenny piece and the tin-foil con-stitutes a circular slit. Let it replace the needle, andexamine the flame of the spirit lamp charged with saltthrough 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 passingthrough the spectroscope being only red, or yellow, or. spectroscope ./, slit; t /, scale illumin- flames to be ; /, 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 imageof the slit in the appropriate part of the spectrum, andthat the image thus built up will take the form of a lineor circle, accortling to the slit we use. Many chemical substances, salts or various metals,become luminous by inserting them into llames, as wehave treated tomniDn salt (chloride of sodium). Witheach metal tin- colour imparted to the flame is resulting spectrum is called a discontinuous spectrum,because it is only here and there that images of the slitare produced ; because some coloured rays, and not all,are present. February i6, i NA TURE 573
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