1214 



Megadiascope. 



Interference Mirror. 

 Fig. 20. 1 : 3. 



Fresnel's Micrometer Ocular. 

 Fig. 22. 1 : 2. 



6 



Diffraction Phenomena with narrow Apertures. 

 Fig. 21. 1 :3. 



The open objective is focussed so that a sharp image of the diaphragm of the object holder 

 is obtained on the screen. If, now, a double refracting prism be brought in front of the objective, two 

 circles of half the brilliancy appear on the screen, these circles rotating around each other when the 

 prism is rotated. If now a gypsum slab of suitable thickness is placed on the object holder, the 

 two circles appear on the screen in the complementary colours. If after removing the gypsum slab 

 the second double refracting prism be introduced into the mount intended to take the double refracting 

 prisms, on turning this prism in quadruple alteration one, two or four circles are obtained on the screen, 

 and on replacing the gypsum slab in the object holder the circles appear in the complementary colours 

 and on combining to one circle this appears white and with the maximum brilliancy. 



10. Demonstration of Interference Phenomena (Figs. 2022). 



A micrometer slit is placed in front of the condenser, the former being adjusted to 1 / 2 millimeter 

 (0.019 inch) width. At 50 cm. distance from this the interference prism is set up. The highly 

 coloured interference bands appear plainly on the 2 3 m. distant screen. 



If instead of the interference prism the interference mirror is placed on the optical bench, the 

 slit must be so regulated that the two mirror images on the screen overlap. The mirror is fixed on 

 a slider the column of the slider being laterally adjustable so that the mirror may be adjusted in such 

 manner that the light leaves the first mirror at a very obtuse angle. 



The diffraction phenomena with narrow openings, gratings and double gratings (F i g. 21) 

 can be demonstrated both in an objective and subjective manner with the aid of the Fresnel micrometer 

 eyepiece (F i g. 22) or a magnifying glass. 



In Fig. 21: 



No. 1 shows the bands resulting from the passage of the rays through two parallel slits. 

 No. 2 shows the bands which result when the rays encounter the edge of a screen. 

 No. 3 shows the bands which result when the rays encounter a thick needle in a slit. 

 No. 4 shows the bands produced when the rays encounter a hair in the slit. 

 No. 5 when the rays encounter a thick opaque thread. 

 No. 6 shows Grimaldi's experiment when the rays pass through a round hole with a black or white 



point in the centre, and according to the distance of the screen. 

 No. 7 shows the displacement of the bands on the insertion of a sheet of mica. 



A concise description of the arrangement of the experiments is appended to the apparatus. 



Cl. 312, 311, 310. 



