ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 35 



in pi. III., figs. 17, 19. Where a is the track of the direct incident 

 pencil, and I, c, d, e, /, g, the tracks of the six diffraction pencils 

 seen at the extreme peripheral margin of the objective image. 

 The field of view takes in three pencils only (central and any two 

 diffraction pencils situate next each other) when oblique illumination 

 is used. The dotted circle of fig. 17 shews this. The several 

 directions of the striae of PI. ang. are given in fig. 18, i, 2, 3, 4, 5, 

 6, corresponding to the positions h, c, d, e, /, g. Two sets of striae 

 equally strong and equally inclined to the axis cross each other at 

 angles varying from 53"-' to 58°. A third slightly less visible 

 crosses the axis at right angles, forming with the inclined striae 

 angles of 60*^ to 6^^'^. The interference of the seven pencils, 

 theoretically considered, gives as result the well-known pattern 

 (fig. 16,) consisting of a central bright area, the maximum of light 

 surrounded by shade lines of minimum light, forming the well- 

 known hexagon whose two longest sides run parallel with the 

 axis (mid rib.) The dirk lines which appear straight with low 

 amplifications show as zigzag lines. These represent the two sets 

 of inclined lines which consist of portions running alternately 

 in longitudinal and oblique directions, whilst the cross lines are 

 formed alone by the obliquely directed portions (see the darkened 

 lines in fig, 16.) This effect may be calculated in each detail, as 

 also the somewhat modified effect seen when oblique illumination is 

 used, from the interference of the pencil a with any two first 

 diffraction pencils situated next each other. But theory shews 

 further — (istly) that three additional lines of striation halving the 

 angle between the first set of striae must arise when onlg two 

 diffraction pencils not situate next each other are brought into 

 play 5 as, for example, ^,y, or </, /, (pi. III., fig. 19.) The striation 

 which then appears is directed at right angles to the line joining 

 the interfering pencils, and the distance between its lines is less in 

 the ratio of ^3 • i than the interspaces of the first set of lines. 

 This system of striae may be best rendered visible by oblique 

 illumination with strong lamp light and full breadth of beam, so 

 that the whole spectrum shifts sidewards, and the two not next 



