72 



seem that a star of the fifth magnitude became invisible with a 

 three-feet telescope when at a distance of six minutes from the 

 planet ; whilst by the latter the same star, after having undergone 

 occultation by the planet, could not be perceived with a large tele- 

 scope till Mars had receded from it a distance equal to two thirds 

 of his own diameter 5 although with the same instrument stars of 

 similar magnitude might be easily distinguished even when in con- 

 tact with the moon's limb. 



As opposed to these observations, the author advances his ow T n. 

 One, dated Blackman-street, February 19, 1822, in which a star of 

 the ninth magnitude as seen with the five-feet equatorial suffered no 

 diminution of its apparent magnitude, at a distance of 103 seconds 

 from the planet. A second, on the night following, when the star 

 42 Leonis having been seen within a second of a degree of the 

 planet's limb prior to occultation by the planet, was perceived after 

 emersion, when only one second and one tenth from it ; the instru- 

 ments of observation in this instance were the five-feet equatorial and 

 the thirty-inch Gregorian reflector, the former instrument being used 

 by the author, the latter by Mr. Henry South. The third was made 

 at Campden Hill, on the 17th of March of the present year, with an 

 eight-feet achromatic of six inches aperture; and in this the star 

 37 Tauri was with a power of 320 seen actually touching the planet's 

 limb. — The star in neither instance suffered more diminution of 

 brightness than might fairly be attributed to the diffused light of 

 the planet. 



From these observations, and the apparently contradictory ones 

 of Cassini and of Rcener, the author of this paper infers, that the 

 existence of the extensive atmosphere of Mars is a subject highly 

 meriting further investigation. 



He then directs attention to the fact that 37 Tauri was of a red 

 colour when in contact with Mars ; whilst 42 Leonis was under 

 similar circumstances of a blue colour: and, from inferences de- 

 pendent upon observation, states, that the apparent anomaly is 

 easily reconcilable, and that an hypothesis is not wanted to account, 

 on the occasion alluded to, either for the red colour of the one star, 

 or the blue colour of the other. 



A paper was read " On the Inflexion of Light." By John Bar- 

 ton, Esq. Communicated by Davies Gilbert, Esq. V.P.R.S. 



The design of the author in undertaking the experiments of 

 which he gives an account in the present paper, is to carry on the 

 investigation of the phenomena of the inflexion of light from the 

 point at which it was left by Newton. He begins by examining 

 these phenomena in their simplest form, comparing the appear- 

 ance of the shadow of an opaque body on a screen of white paper 

 at different distances, with the appearance it would exhibit if the 

 rays passed by the edge of the body, without suffering any deviation 

 from a rectilinear course. It is well known that, under these cir- 

 cumstances, the real shadow is broader than the geometrical 

 shadow, indicating a deflexion of the rays from the edge of the 



