i r>4 Notes, 



they happen to fall with respect to the intensity curve of the single 

 slit, will show a corresponding change of phase. 



Thus, not only are the relative intensities of the maxima deter- 

 mined by the intensity curve of the single slit, but we may also 

 draw certain conclusions from the same as to their phase. 



When, as in a Microscope, using parallel light from the con- 

 denser, we have the spectra produced by the object grating formed 

 in the back focal plane of the objective, we can detect any change 

 of phase from the normal by an alteration of the position of the 

 lines in the image or view-plane. By normal phase-difference I 

 mean that difference which occurs in consequence of the position 

 of the grating in the object-plane, and which is the cause of the 

 images of the lines being formed at the points where, according to 

 geometrical optics, they should be formed.* 



When the position of the grating on the Microscope stage is such 

 that one of its slits is situated symmetrically on the axis of the 

 Microscope, this phase-difference is nil ; when it is situated so that 

 one of the bars is situated symmetrically on the optical axis, the 

 normal difference between two spectra would be half a phase- 

 period. It is essential, however, not to confuse this particular 

 phase-difference with that pointed out by Mr. Conrady, the latter 

 being an entirely different and additional effect. 



We now come to the photographs. 



Fig. 6 shows the image of a reciprocal grating (magnification 

 X 13), photographed in the ordinary manner. In the upper half, 

 the width of the slits to that of the bars is as 1 to 2. In the lower 

 half, the width of the slits to that of the bars is as 2 to 1. It will 

 be seen that a bright line in the upper half always corresponds in 

 position with a dark one in the lower half, and vice versa. 



Fig. 7 is the central part of the diffraction pattern produced by 

 the grating in the back focal plane of the objective (monochromatic 

 light having been used). It shows the central or zero maximum 

 and the first and second maxima on both sides. 



Fig. 8 illustrates diagrammatically the intensity curves pro- 

 duced by the upper half and those of the lower half of the grating, 

 the dotted lines showing the intensity curve of a single slit in the 

 upper half and in the lower half. It will be seen that, though the 

 positions of the maxima in both cases coincide, the intensity curves 

 of the single slits are different — that of the slit in the upper half 

 being twice as wide as that of the slit in the lower half, because the 

 former slit is half as wide as the latter. It will further be noticed 

 that, owing to this fact, whilst the first maxima furnished by the 

 two gratings both occupy a position between A and B on the 



* See R. T. Glazebrook, " Note on the Diffraction Theory of the Microscope, as 

 applied to the case when the Object is in Motion," Journal Physical Society of 

 London, 1904, pp. 157-9. Also J. D. Everett, "A Direct Proof of Abbe's Theorems 

 on Microscopic Resolutions of Gratings," this Journal, 1904, pp. 385-7. And also 

 J. Rheiuberg, " On the Influence on Images of Gratings of Phase-Differences amongst 

 their Spectra," this Journal, 1904, pp. 388-90. 



