388 



NOTES. 



On the Influence on Images of Gratings of Phase Difference 

 amongst their Spectra. 



By Julius Bheinbeeg. 



It is my privilege to bring an experiment to your notice this 

 evening * which illustrates very well the effect on the image of a 

 grating which is produced by alterations of phase between the 

 spectra which it forms in the back focal plane of the objective. 



In Prof. Everett's interesting paper we have heard how it 

 occurs, that when a grating is moved in its own plane at right 

 angles to the lines, a change of phase is brought about amongst 

 these spectra, and that this change of phase accounts for the lines 

 shifting in the image, although the spectra in the fecal plane do 

 not shift their position. Similar results have, I think, recently 

 been arrived at by Dr. E. T. Glazebrook. 



Now it occurred to me, that if the shifting of the lines in the 

 images were caused by change of phase amongst the spectra, we 

 ought to be able to make the lines in the image of the grating move 

 without moving the grating on the stage at all, by the expedient 

 of introducing a difference of phases amongst the spectra in the 

 back focal plane. 



This is the experiment I have to show you. The arrange- 

 ments are as follows. The Microscope used is one of the so-called 

 " Demonstration " microscopes specially designed by Prof. Abbe 

 many years ago for his experiments showing the relation between 

 diffraction effects and image formation. I am indebted for the 

 loan of this apparatus to the firm of Carl Zeiss. Without entering 

 into details about the many ingenious arrangements in this instru- 

 ment, it suffices our purpose that the back focal plane of the 

 objective is readily accessible, and that the spectra formed there 

 and the resulting image in the view-plane can be alternately 

 examined with rapidity and ease. 



The grating is placed on the stage of the Microscope. In the 

 substage condenser we use a narrow slot, and in the back focus of 

 the objective, where the grating forms the spectral images of this 

 slot, we introduce an Abbe Glass Wedge Compensator.! We 



* See account of the Proceedings of the June Meeting at end of this Number. 



t To understand the action of the compensator, think of a square slab of glass, 

 the faces of which, instead of being accurately plaue parallel, are very slightly wed^e- 

 ehaped. Imagine the square of glass cut in half, and that we have two similar 



