764 SUMMARY OF CURRENT RESEARCHES RELATING TO 



within a small fraction of a fringe, which of course corresponded to the 

 same small fraction of the grating space in the replica itself. A replica 

 corrected in this manner gives the same resolving power as the grating 

 from which it was made within a very few per cent., provided the glass 

 on which it is mounted is optically perfect. In order to transform one 

 of these replicas into a metal grating it is necessary to find a substance 

 which will fill up the grooves (fig. 110) between the replica and the 

 glass surface after the replica has been corrected and dried, and which 

 will adhere to the glass sufficiently to allow the replica to be stripped 

 off. The result will then be similar to fig. 111. This is evidently a 

 fair copy of the grating from which the replica was made. It was acci- 

 dentally found that certain gums dissolved in the collodion solution will, 

 on gently heating the glass plate upon which the corrected replica is 

 placed, slowly ooze out, filling up the grooves as indicated, and on 

 cooling will harden, and allow the replica to be stripped off. The 

 resulting grating (fig. Ill) may now be treated in one of the two 

 following ways :— (1) It may be covered by a thin film of platinum, 

 nickel, or other suitable metal in a vacuum, which coating may be 

 improved by subsequent electroplating, thus producing a durable metallic 

 grating having a perfect optical surface. (2) It may be treated with 

 hydrofluoric acid gas, thus transforming it into a glass or quartz trans- 

 mission grating with an equally good optical surface, as the gum used 

 is not affected by the acid, while the glass or quartz between the ridges 

 is rapidly attacked. The method is equally applicable to concave 

 gratings. The author has a number of plane platinum, nickel and gold 

 gratings made by the above process, which perform admirably, as well 

 as a number of glass ones made by the hydrofluoric acid process. 



Use of the Grating in Interferometry.*— C. Barns points out that, 

 on replacing the symmetrically oblique transparent mirror in Michelson's 

 adjustment by a glass grating, it is possible with ordinary plate glass 

 and a non-silvered grating to produce interferences between pairs of 

 diffracted spectra, if returned by nearly equidistant mirrors to a telescope 

 in the line D. Both of these spectra are very brilliant, and not very 

 unequally so, and the coincidence of spectrum lines, both horizontally 

 and vertically, brings out the phenomenon. This phenomenon is of 

 the ring type, but it occupies the whole field of the spectrum from red to 

 violet. Brilliant large confocal ellipses with horizontal and vertical 

 symmetry are obtained, and the spectrum* lines, simultaneously in focus, 

 may serve either as major or minor axes. Their interferometer motion 

 is twofold in character, consisting of radial motion combined with a 

 drift of the figure as a whole in a horizontal direction. Naturally, a fine 

 slit is of advantage, but the experiment succeeds with a wide slit, 

 especially in the red, even after spectrum lines vanish. 



Dark-ground Illumination with High Powers.f — R. F. Jones re- 

 marks that, "The ability to see living bacteria without the tedious process 

 of preparation and staining, by the use of one of the many dark-ground 

 illuminators that are uow obtainable, has to a large extent revived interest 



* Amer. Journ Sci., xxx. (1910) pp. 161-71 (2 figs.), 

 t Knowledge (1910) p. 284. 



