1279 



When dried, this formed a sufficiently opaque tightly stretched screen. 

 With a fine needle small apertures were made in it. In order to 

 vary the magnitude of these apertures the needle was fixed in a 

 round holder. This holder fitted into a cylindrical tube in which it 

 could be slided up and down and be fixed in a definite i)Osition. 

 This enabled us to make all apertures equally wide and also to 

 alter their width (by protruding a longer or a shorter part of the 

 needle from the cylinder). 



In the second method 1 worked with glass-plates covered with 

 lycopodium powder or with glass-plates on which small mercury 

 spheres had been precipitated by evaporation. The mercury can 

 easily be precipitated in sphei-es of ^^^ "^'^i diameter or less. They 

 are however not all of the same magnitude. Three such plates were 

 in my possession. 



The first method offers many advantages. The phenomenon can 

 be continuously followed with the eye while the number of aper- 

 tures is increased, while in the case that the refracted light has the 

 same direction as the incident beam, the latter does not blind as in 

 the other methods. Visually the details of the phenomenon could be 

 observed much quicker, more accurately and more easily than 

 photographically, while the occurrence of colour-phenomeua did not 

 remain unobserved. 



The source of light was an arc-lamp at a distance of about 5 m 

 from the objective of the telescope. The lamp had been screened off 

 as perfectly as possible, while the apertures in a screen of lead 

 that could be rotated formed the light-points. In order to vary the 

 experiments there were apertures of 0.5, 0.4, 0.3, 0.25, 0.15 mm 

 diameter. 



Beginning with a small number of apertures in the screen of 

 black paper and passing gradually to a greater number, we see 

 how the diffraction image of a few apertures, which agrees with 

 the classic theory, changes into the phenomenon of the fibrous 

 structure. This change is very striking. 



Working with natural light we see with two holes a small 

 number of diftraction lines, the middle one of which is uncoloured 

 and passes through the point 0, where the image of the considered 

 light-point is formed. This point will be called the centre. On l^oth 

 sides of the middle and most intense line a small number of lines 

 is observed, which are altei-nately light and dark, and which show 

 the spectral colours. Let the system of lines round the centre be 

 denoted by A, the uniformly illuminated field on both sides by B. 



We must remind that according to § 3 all this is superposed 



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