56 On a new Proposition in the Theory of Diffraction. 



equality of the energies is proved as soon as this principle is 

 verified for one of these apertures. 



The observations necessary for the purpose were carried out 

 by the following method. In the vicinity of the focus of the 

 collimator C C (fig. 2) was placed a portion of a spherical sur- 



Hff. 2. 



7 



1 



Si I | A W 



face F F made of fine paper oiled, the axis of symmetry of 

 which fell into that of the collimator; behind FF was a petro- 

 leum-lamp burning with a large steady flame, which very 

 equally illuminated the surface F F (the slight differences of 

 intensity at its margin have no influence on the observation, 

 since for these $(«, jS) already vanishes). The apparent mag- 

 nitude of F F for the -middle of the collimator amounted to 

 about 8°; therefore its dimensions were quite sufficient. 



In front of one of the half-lenses, H 1? of a heliometer a 

 square aperture was brought, and remained there during the 

 entire observation ; before the other half-lens, H 2 , plane- and 

 space-apertures were consecutively fixed, of the most varied 

 shapes, as well as consisting some of one and others of a plu- 

 rality of parts, and their dimensions and position were accu- 

 rately determined. Both the half-lenses remained fixed during 

 the observation ; and their axes coincided exactly with that 

 of C C. 



Before the focus of the heliometer the rotating Nicol, N 1; 

 polarized the diffracted light ; and the circular diaphragm o n 

 placed in the focus of the ocular, permitted only the middle 

 part of the diffraction image to be observed, while the parallel- 

 plane glass plate P, inclined 45° to the axis of the tube, 

 reflected the image of the small circular aperture o n exactly in 

 such a way that o / and the image of o n in the ocular stood in 

 immediate juxtaposition. The second Mcol, N 2 , placed before 

 the ocular, retained its position unaltered during the obser- 

 vation. 



In the observation properly so called, one half-lens of the 

 heliometer was covered and the other left uncovered ; and 

 now Ni was rotated until the images of o / and o tl visible in the 

 ocular both possessed the same intensity ; the former half-lens 



