[dawes] 



DISPERSION BY INTERFERENCE METHOD 



285 



dispersion of the glass plate. Certain constituents of white light are 

 destroyed by interference and others reinforced and, in general, there 

 are present so many constituents so widely distributed throughout the 

 spectrum that the light is white to the eye and there is no sign of 

 interference. It is possible, however, with very thin plates to adjust 

 D so that only sufficiently few constituents are present that coloured 

 fringes may be observed. (See above reference.) In order to show 

 the presence of interference fringes with thicker plates of glass one 

 must use a spectroscope to analyse the light. 



The fringe system is situated at infinite distance, the photographic 

 objective L of focal length / produces an image of the system at its 

 second focal plane, the image being a series of concentric superposed 

 circles. S is the slit of a single prism spectrograph of which the focal 

 lengths of collimator and camera lens are /' and /". (Camera may 

 be replaced by a telescope for visual observation.) The slit is at the 

 focal plane of L so that the fringe system is focussed upon it. It is 

 much longer than for ordinary use in order to produce a relatively 

 wide spectrum. The condensing lens is so adjusted as to give illumina- 

 tion over the whole length of the slit. The spectroscope thus selects a 

 narrow section of the fringe system at a vertical diameter of the set 

 of circles and analyses it. The central longitudinal element of the 

 spectrum corresponds to light traversing the interferometer system 

 normal to the mirrors and the faces of the plate. Other longitudinal 

 elements correspond to rays parallel to the vertical plane but inclined 

 to the horizontal as indicated in Fig. 2, which is a vertical section of 

 the system. Plates 1-8 show the character of the spectra observed 



Table 1 



Avith various values of D in arithmetical progression as recorded in 

 Table 1. Readings on the negatives were taken by an optical com- 

 parator graduated to 1/100 mm. and interpreted into wave-lengths 

 by means of a calibration table. (Section 7.) 



