38 ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 



Fig. II, lines crossing at 60°. In lig, 10 the network has equal sided 

 meshes at a and b, but at c and d, the sides of the meshes are as 3 : 2, and the 

 longer side runs horizontally at c, vertically at d. In fig. 1 1 the grating crosses 

 with equal sided rhombic meshes at a, and b, but unequal sided meshes at c, 

 and d. 



Fig. 12, shows the effect of diaphragm i.c, upon the group of lines crossed 

 at 90^, when the slit in the diaphragm is inclined at 45^ from the vertical 

 towards the left side. All the real lines disappear and diffraction lines appear 

 everywhere inclined at right angles to the slit, and therefore diagonally to the 

 actual lines. See experiment IV. 



Fig. 13 shows the effect of diaphragm i.c, on the crossed grating of fig. 11. 

 See experiment V. 



Figs. 14 and 15 show the effect of diaphragms i.f, and i.g, upon tlie 

 respective groups of crossed lines, figs. 10 and 11. See experiment VI. 



Fig. 16, the pattern of markings on Pleurosigma angm. 



Fig. 17 shows the diffraction spectrum of PI. angm., and what portion of it 

 is operative when oblique illumination is employed. 



Fig. 18 shows the several directions of the strias of PI. angm. corresponding 

 to positions b, c, d, e, f, g, fig. 19. 



Fig. 19 shows the mode of employing two diffraction pencils, not situate 

 next to each other with the central pencil, and use of oblique illumination. 



