84 



Between me initial and the inuxiiiiiiin value of '/, there arc Itiit 

 three ordiiiates and so, the scaled axes have three dividing-points 

 and the image has three interference-enrves, each consisting of four 

 ecjiial parts (Pig. 2). 



In the fornuila, '/ — <r' increases fi-om 15° to 30° and the phase- 

 difference it—n' from — 15° to -|-15°. 



The constrncted curves inaj be compared to the experimental 

 curves in fig. 3, obtained by superposition of two equal unissons. 



o 



!■ I » > — " « I I I I 1 I >- " 



O I Z 3 1^ S 4 y & ^ 10 /I iz i3 ,v tS li '7 iS 'J t^ tj U Zi ^'^ 



Fig. 1. 



each containing 12 ellipses. A much fiïier result is obtained with 

 unissons of f.i. 50 ellipses, or bj comparing to constructed unissons 

 ' in superposition — tiiese drawings, however, require much time. 

 It will be evident, that an image with more interference-curves 

 may be obtained by interpolating a same number of curves between 

 two succeeding curves of fig. 2. 



Construction of the Lemniscates. 



This construction is more difficult than that of the hyperbola's, 

 because the image, going to the centre, shows three different species 

 of curves, viz.: ovals, flattened ovals and hyperbola's with doubled 

 ovals. 



Only the outer curves are seen in the case of few isophasic lines ; 

 they are as easily to construct as the hyperbola's, viz.: by joining 

 the points of intersection, but now following the other diagonal 

 .(fig. 4), according to a phase-difference, that begins with 90°. 



The constructed curves of fig. 4 may be compared to the expe- 

 rimental curves of fig. 5, the unissons having 12 ellipses each. 

 Fig. 5 is somewhat irregular, owing to the small number of ellipses. 



A new difficulty arises from observing, that the axes of co-ordi- 

 nates are not axes of symmetry for the image of the lemniscates, 

 as is required in the found formula. Still, this image was built in 



