THE PHYSICAL BASIS 



by only two methods, dispersion and diffraction. When white light 

 is passed through a glass prism, as in Sir Isaac Newton's original experi- 

 ment, a spectrum is obtained. Only under certain, now well-defined 

 conditions is such a spectrum pure, i.e. the colours do not overlap. It 

 is commonly said that the white light is " split up " into its component 

 parts, which are coloured. Lord Rayleigh has given sound reasons for 

 the view that white light is not thus analysed into component parts, 

 but that the periodicities characteristic of the several rays are in reality 

 imposed by the prism and are not antecedently present in the white 

 light. Be this as it may, dispersion of white light by prisms enables 

 us to obtain coloured light in a pure state. By passing white light 

 through a diffraction grating a pure spectrum can also be obtained. 

 This method has the advantage that the deviation of the component 

 rays varies within narrow limits directly with the wave-length, i.e. equal 

 differences of wave-length are separated by equal distances in the 

 spectrum. It suffers, however, from the disadvantage that the spectrum 

 is less bright and less extended than the prismatic spectrum, and from 

 the still greater objection that the interference spectrum is never free from 

 scattered Kght. In the prismatic spectrum the dispersion increases as 

 the wave-length diminishes, so that the violet end is much more extended 

 than the red end and its intensity is diminished. Moreover the amount 

 of dispersion depends upon the character of the prism or prisms employed. 

 Hence it is necessary for accurate observations that each prismatic 

 spectrum shall be calibrated. The Fraunhofer lines, being absolutely 

 constant in situation, afford a series of fixed points from which the 

 calibration curve of the given spectrum can be constructed^. The 



o 



A. U. 



A = 7606 in extreme red. 

 B = 6869 in deep red. 



^ Burch, Practical Exarcincs in Physiological Optics, p. 102, Oxford, 1912. 



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