32 ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 



On replacing the eyepiece we find, in accordance with this 

 arrangement, that the wide set of lines is still visible, whilst the 

 set of narrow lines is no longer visible, (pi. II., fig. 8.) It is thus 

 proved that a single direct image does not resolve the lines, for 

 without any further change than is obtained by turning the slit 

 into the crossed position and thus admitting additional images, the 

 group of narrow lines at once re-appears. 



The next experiments are made with the groups of crossed 

 lines, (fig. 10 and ii.) A description of the mode in which these 

 are prepared has been already given. The grating with square 

 meshes has at a and h equal sided meshes, but the sides of the 

 squares are as 3 : 1 ; at c and d the meshes have one side related 

 to the other as 3:2, but the longer side runs horizontally at c, 

 vertically at d. The grating crossed at an angle of 60** has at 

 a and b regular equal sided rhombic meshes, while the meshes at 

 c and d have sides of unequal length, the longer side lying vertically 

 at c but inclined at d. 



With these groups of crossed lines the diaphragm cards i c, i d^ 

 must be used when it is desired to render secondary systems of 

 lines visible. The experiments are rather difficult to make 

 successfully, by reason of deficiency of light, when the beam of 

 illuminating rays employed is so small that the spectra separate 

 from each other, and thus allow the position of the card slit, in 

 respect of these spectra, to be directly seen. This is assuming 

 that the illumination is effected not as in the preceding experiments, 

 but with plane mirror and the lamp at sufficient distance. If, 

 however, the direct observation of the spectra (by looking down 

 the tube) be given up, a large beam of light may be used, such as 

 will fill the whole aperture of the objective. In this case each 

 effect is produced in all the experiments just as described if the 

 diaphragm cards occupy their proper position, and the images will 

 be clear and brightly defined. At first view this may appear 

 strange, but it results from the principle of Professor Abbe's 

 theory that the images arise from the interference of conjugate 

 groups of rays, i.e., of rays generated by diffraction of the incident 



