November 2, 1905] 



NA TURE 



there is little or no danger to the operator through faulty 

 switch-gear. 



The switch-board proper is carried by three galleries 

 extending the whole length of the north side of the engine- 

 room and continued along the east end. The control board 

 is on the middle gallery and projects slightly, so that the 

 operator has a clear view up and down the engine-room. 

 From the switch-board the energy is distributed to the 

 various substations situated at various points along the 

 system, and it is there converted to low-voltage direct 

 current at 550 volts, and thence distributed to the live 

 rail. Throughout the whole station it is remarkable to 



38 grains of Anthony's pure snowy cotton in 2.\ ounces of 

 pure amyl acetate, precipitating the resultant collodion in 

 a large tray of pure water — constantly agitating the 

 mixture — thoroughly drying the precipitate, and then re- 

 dissolving it in the same quantity of pure amyl acetate. 

 The collodion thus obtained is carefully filtered, and is 

 then ready for use. 



The grating to be copied is levelled in a roomy drying 

 cabinet, which, in order to preclude dust particles, should 

 be as free from draughts as possible, the surface dusted 

 with a soft camel-hair brush, and the collodion flowed 

 ever it evenly. The author uses about twenty-five drops 



K W. I ieneralcr 



find the extent to which labour-saving devices are 

 employed. 



Thanks are due to Mr. Chapman, general manager and 

 chief engineer, for permission to view the station, and to 

 the Institution of Electrical Engineers for the accompanying 

 illustration of the armature of one of the generators. 



REPLICAS OF DIFFRACTION GRATINGS. 



I7ROM an article in No. 2, vol. xxii., of the Astro- 

 physical Journal, we learn that Mr. R. J. Wallace, 

 of the Verkes Observatory, has attained great perfection 

 in the production of replicas from plane diffraction gratings. 

 After some amount of previous research, he decided on 

 following Thorp's method in its essentials with several 

 modifications which his experience suggested. Mr. Thorp 

 first flooded his original grating with high-grade oil before 

 pouring on the celluloid solution on which the replica was 

 made. Mr. Wallace found it better to omit the oil. In 

 the original method a solution of gun-cotton in amyl 

 acetate with camphor added was employed as the material 

 for the replica, but Mr. Wallace found that he could obtain 

 much clearer and brighter copies by not adding the 

 camphor. His successful solution is made by dissolving 



NO. 1879, VOL. -3] 



of the solution in copying a 2-inch grating. The grating 

 is then replaced on the levelled support and left to dry 

 for about eight to twelve hours ; the longer the drying 

 period the better is the resulting copy. After being 

 thoroughly dried the grating is placed in pure distilled 

 water it normal temperature together with the glass 

 (" white optical crown ") support, which has previously 



I ] evenly coated with the adhesive medium, plain hard 



gelatfn. After a few minutes' soaking the edge of the 

 film may be sprung from the grating, and the whole of 

 it is then detached and immediately placed on the 

 previously prepared gelatin surface and clamped there. 

 Perfect contact is obtained by drawing a piece of the softest 

 velvet rubber very lightly over the surface in the direction 

 of the length of the lines. 



The contraction suffered by the replica during the 

 twenty-four hours' drying period slightly alters the number 

 of lines per inch, but the effect is very small. In some 

 of Mr. Wallace's copies this alteration produced 572 lines 

 per mm. instead of the 568 lines that occupied the same 

 space on the original. Two reproductions of the solar 

 spectrum, one taken with the original grating, the other 

 with the copy, show the resulting increase of dispersion 

 caused by the contraction, and also show that everything 

 which is resolved by the original grating is also resolved 

 equally well under the same conditions by the copy. 



