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the distance between l and the plane of the slit. When, as in the 
experiment, the path of the light between the slightly silvered mirror 
and the lens is about 600 em., we can by a displacement of 4.5 em. 
(focal distance of the projecting lens 50 em.), observe two sharp, 
coherent image points lying vertically above each other, in the 
image plane. The distance of the image points is a function of the 
wavelength. When they are projected on the slit of the spectroscope, 
the dependence of image distance and colour is directly observed. 
Fig. 3 represents a positive photo obtained with the tube of sodium 
under the said circumstances. The dark line on the left side and 
the dark line on the right side originate from one luminous point, 
the faint lines from the other. The difference of intensity of the 
lines was caused by this that the two luminous points were not 
thrown on the slit exactly under each other. The lines of Fig. 3 
could not be observed in the neighbourhood of the absorption D- 
lines. In the lefthand part of the figure our lines are both curved 
upwards, in the righthand part both curve downwards. It is further 
noteworthy that the lines in the lefthand part intersect, whereas in 
the righthand part they diverge more and more from each other. 
When we focus again the interference fringes upon the slit of 
the spectroscope we see the interference figure of Fig. 2 in the 
spectroscope. 
Now it seems rather plausible that the existence of Fig. 2 is due 
to the shape of the lines of Fig. 3. We then expect a shift in the 
central part of the figure of the whole system of fringes, on the left 
upwards, on the right downwards. Further we expect interference 
fringes close together where the lines of Fig. 3 diverge greatly from 
each other, widely apart where the distance of these lines is small. 
No interference fringes are expected where the lines intersect. Thus 
the typical rhombic central part would arise in the figure. 
It should still be pointed out that there is a narrow dark horizontal 
line in Fig. 2, this is due to a particle of dust on the slit. The 
vertical shadows in the lefthand part of the figure are the first 
indications of the absorption band spectrum of sodium. 
