196 Lord Rayleigh on Copying Diffraction-gratings, 



electrodes with occluded gases the less Las been the waste of 

 aurae by diffusion &c. During the passage of the current, 

 the aurae and the occluded gases are used up by chemical 

 action, just as the zinc is in an ordinary cell ; and as their 

 quantity diminishes, the effective E.M.F. of the voltaic couple 

 formed by the plates also diminishes. 



If through leakage of the apparatus, or other causes, at- 

 mospheric air obtains access to the liquid surrounding the 

 — electrode, the dissolved oxygen thence resulting will ofjeourse 

 tend to reduce the aura of hydrogen by a chemical action of 

 a kind akin to the " local action " of an ordinary voltaic 

 element, i. e. not contributing to the work done by the 

 current generated. In this case the rate of fall of the u sub- 

 sequent polarization " (the electrode not being connected by 

 a wire so as to produce a current) will be more or less en- 

 hanced above the rate due to diffusion only, as above described 

 (§ 70) ; whilst if the electrodes are connected and a current 

 is made to pass, the E.M.F. of the voltaic couple thus produced 

 will diminish more rapidly than would be the case were no 

 dissolved oxygen present in the liquid round the hydrogen- 

 plate, just as the zinc of an ordinary cell wastes more rapidly 

 when there is local action than it does when there is not, other 

 things being equal. 



XX Y. On Copying Diffraction-gratings, and on some Pheno- 

 mena connnected therewith. By Lokd Bayleigh, F.RS., 

 Professor of Experimental Physics in the University of 

 Cambridge* 



IN the Phil. Mag. for February and March 1874 I gave an 

 account of experiments in the photographic reproduction 

 of gratings ruled with lines at a rate of 3000 and 6000 to the 

 inch. Since that time I have had further experience, extend- 

 ing to more closely ruled gratings, and have examined more 

 minutely certain points which I was then obliged to leave 

 unexplained. The present communication is thus to be re- 

 garded as supplementary to the former. 



Some years ago Prof. Quincke described an unphoto- 

 graphic process by which he had succeeded in copying en- 

 graved glass gratings. He began by depositing a thin 

 coating of silver by the chemical method upon the face of the 

 grating. The conducting layer thus obtained was then trans- 

 ferred to an electrolytic cell, and thickened by the deposit of 

 copper, until stout enough to be detached from the glass 

 substratum. In this way he prepared an accurate cast of the 

 * Communicated by the Author. 



