73 



of correcting or disregarding certain discrepant appearances or ir- 

 regularities in the organ of vision ; as a further illustration of which 

 I may cite the fact, mentioned by Mr. Warren De la Rue in his 

 " Report on Celestial Photography," that the retinal image of a star 

 is, at least under some atmospheric conditions, made up of " a great 

 number of undulating points," which, however, the mind rightly 

 interprets as the effect of the presence before the eye of a single 

 minute object. That this corrective power is, as might be supposed, 

 very limited, may be proved experimentally by this instrument ; for 

 if the small ends be enlarged in only a slight degree, so as to increase 

 this slurring on the retinae, a very marked diminution in clearness of 

 definition is the immediate result. 



That form of the stereotrope, in which Professor Wheatstone's re- 

 flecting stereoscope is made use of, and which is better adapted for 

 the exhibition of movements that are not only local but progressive 

 in space, it is needless to describe here, because the principles it in- 

 volves are essentially the same as those which are stated above. 



III. " On the Lunar-Diurnal Variation of the Magnetic Declina- 

 tion obtained from the Kew Photograms* in the years 

 1858, 1859, and I860." By Major-General EDWARD 

 SABINB, R.A., Treas. and V.P.R.S. Received December 

 19, 1860-J-. 



Having communicated to the Royal Society in a recent paper an 

 analysis of the disturbances of the declination in the years 1858 and 

 1859, shown by the photograms of the Kew Observatory, I propose 

 in the present paper to submit the results of the lunar-diurnal 

 variation of the declination in the years 1858, 1859, and 1860, 

 obtained from the same source. The directions of the declination 

 magnet at the instant of the commencement of every solar-hour 

 having been tabulated from the photograms, and the final normals 



* The term Photogram is adopted in place of Photograph in conformity with 

 modern usage. 



t [Note added on February 8th, 1861.] When this communication was read to 

 the Royal Society on January 10th, 1861, it contained the lunar-diurnal variation 

 for the years 1858 and 1859 only : whilst it was passing through the press, the 

 calculation of the lunar-diurnal variation for 1860 was completed, and the results 

 in that year have been added. 



