470 



Prof. G. G. Stokes. On a Simple 



[Feb. 24, 



Cristobal were observed at the time of the earthquake to be covered 

 by clouds of, to all appearance, gaseous vapour ; and the Padre 

 Bravo, Curate of Lilio, asserts, that the movement of Banajoa was so 

 awful to behold that residents of that village, situated at the base 

 of the mountain, feared that it would fall over and bury them 

 beneath it. 



The two sheets containing diagrams of the five principal shocks 

 were lithographed at Manila, under the careful supervision of Father 

 Faura, and I thought it better to send them as received rather than 

 attempt a tracing, the lines being so complicated. I have not 

 appended a translation of the few descriptive notes on the sheets, as 

 the terms used are almost identical with their English signification. 



February 24, 1881. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. "On a Simple Mode of Eliminating Errors of Adjustment 

 in Delicate Observations of Compared Spectra." By Pro- 

 fessor G. G. Stokes, Sec. U.S. Received February 12, 

 1881. 



When the identity or difference of position of two lines, bright or 

 dark, in the spectra of two lights from different sources has to be 

 compared with the utmost degree of accuracy, they are admitted 

 simultaneously into different but adjacent parts of the slit of a spec- 

 troscope and viewed together. It was thus, for instance, that Dr. 

 Huggins proceeded in determining the radial component of the velocity 

 of the heavenly bodies relatively to the earth. It is requisite that the 

 two lights that are to be compared should fall in a perfectly similar 

 manner on the slit : and it will be seen, from a perusal of his paper, 

 how careful Dr. Huggins was in this respect. 



In a paper read before the Royal Society on the 3rd instant, Mr. 

 Stone has proposed to make the observation independent of a possible 

 error in the exact coincidence of the lights compared by constructing 

 a reversible spectroscope, by which the light should be refracted 

 alternately right and left, supposing for facility of explanation the 

 slit to be vertical. 



