2G8 Chronicles of Science. [April, 



might possibly present themselves to the observer. If a lunar atmo- 

 sphere exist, which either by the substances of which it is composed, or 

 by the vapours diffused through it, can exert a selective absorption 

 upon the star's light, this absorption would be indicated to us by the 

 appearance in the spectrum of new dark lines immediately before the 

 star is occulted by the moon. Again, if finely-divided matter, aqueous 

 or otherwise, of the nature of " fog," were present in the moon's at- 

 mosphere (a supposition to which telescopic observation is opposed), 

 or even any considerable amount of invisible vapour, the red rays of 

 the star's light would be enfeebled in a smaller degree than the rays 

 of higher refrangibilities. In this case the blue end of the spectrum 

 would appear to fade, leaving the red rays comparatively undiminished 

 in brightness at the moment of the star's extinction. 



If, however, there be about the moon an atmosphere free from 

 " vapour," but of some density, then, because of the greater refraction 

 which the more refrangible rays of the star's light woidd suffer in 

 passing through it, the blue end of the spectrum would continue 

 visible for a very small interval after the red rays had disappeared. 

 Even if the moon's atmosphere were not of great extent, but sufficiently 

 dense, the spectrum would probably not be extinguished at the same 

 instant throughout its length, but a lagging of the violet and blue 

 rays behind the red would be perceptible. The star examined was 

 e Piseium, at its occultation of January 4 last ; the telescope had an 

 aperture of 8 inches in diameter, and a focal length of 10 feet. In 

 the spectrum apparatus two prisms were employed — one having a 

 refracting angle of 35°, the other of 45°. The spectrum was viewed 

 through a small achromatic telescope of 6 - 75 inches focal length, 

 furnished with an eyepiece magnifying nine diameters. The tele- 

 scope armed with this special apparatus was directed to s Piseium, 

 about five minutes before the almanac time of the occultation, which 

 was 5h. 53m. The clock motion carrying the telescope was carefully 

 adjusted, and by this means the image of the star was kept exactly 

 upon the narrow slit of the spectrum apparatus. During a period of 

 three minutes up to the disappearance of the star, the spectrum was 

 observed steadily and without interruption. On the first point of 

 interest, whether any dark lines additional to those belonging to the 

 star appeared, Mr. Huggins is unable to speak decidedly. The state 

 of our atmosphere was constantly varying, in consequence of which the 

 stellar lines were seen with more distinctness at some moments than at 

 others. A few seconds before the extinction of the star, some lines in 

 the red, which had not been before noticed, were distinctly seen. 

 These lines, however, might have belonged to the star, and have been 

 brought out by a greater steadiness of our atmosphere at that mo- 

 ment. The mode of disappearance of the spectrum of s Piseium can 

 be described with more certainty. It was expected that the spectrum 

 would disappear by a sudden failure of its light ; but such was not 

 the impression produced at the moment of extinction. The appear- 

 ance suggested an opaque screen, equal in length to the spectrum 

 passing before it with a rapid motion in the direction of its breadth. 

 On this occasion the spectrum, as seen in the instrument, was very 



