Mechanical Science. 163 



compartment the direct light from another lamp, employed as a 

 standard of comparison. By adjusting the respective distances of 

 the lamps, the lights on the paper were rendered sensibly equal in 

 point of intensity, the equality being judged of by the eye viewing 

 them from the other side. The measurements were taken alternately, 

 first one of the direct, and then one of the reflected lights, until a 

 sufficient number of uniform results were obtained. The author, 

 after taking every precaution that occurred for insuring accuracy, 

 invariably found that the proportion of light reflected from metallic 

 substances, instead of increasing, diminished in pretty regular gra- 

 dation, as the angle of incidence was augmented. Thus, in the first 

 experiment, when the angle of incidence was 20, the proportion of 

 the reflected to the incident light was as 69.45 to 100 ; at 40 it 

 was 66.79 ; and at 60 it was reduced to 64.91. Some irregula- 

 rities occurred in the series of results deduced from different sets of 

 experiments, arising partly from the variableness of the light given 

 out by the lamps, and partly from the difficulty of preserving the 

 metallic surface in the highest state of lustre which it has when 

 newly polished. The author combats the opinion, that the quan- 

 tities of light which metals are capable of reflecting when polished, 

 are in the ratio of their densities ; and finds that in those metals 

 which were the subjects of his experiments, the quantities of light 

 absorbed or lost by reflection at incidences nearly perpendicular are 

 almost exactly in the ratio of their specific heats*. 



7. ON THE APPARENT PROJECTION OP STARS UPON THE 



MOON'S DISK. 



The attention of astronomers has lately been called in a particular 

 manner, by Sir James South, to the extraordinary effect which had 

 often previously been observed of the apparent projection of the stars 

 upon the moon at the time of occultation. The star, on coming up 

 to the moon, in place of disappearing instantly behind its edge, 

 appears (for several seconds occasionally) to advance a short space 

 on to or before its disk, and then disappear. This effect does not 

 always happen with the same occultation ; some persons see it- 

 others do not; it happens for variable periods of time, and upon 

 both the dark and bright limb of the moon, though most frequently 

 upon the latter^ 



A very curious letter upon this subject has been written by M. 

 Gergonne to the editor of the Bibliotheque U?u'verselle t in which he 

 describes an example of this illusion, of rather an early date. 

 * Before 1789, or rather in 1786, I cannot say at what season, as I 

 was coming after mid-day from the College of Nancy, where I 

 studied, (it consequently was about a quarter to five o'clock,) 

 I found about a dozen men and women in a group, at the bottom 

 of the Rue St. Michel, very nearly in front of an Eglise de Penitens 



* Phil. Mag., N, S., viii, 60. 



