TKANS ACTIONS OF SECTION A. 685 



of the corresponding gas in the atmosphere at the Earth's surface being taken as 

 unity. If we take the density at an infinite distance from the Sun to be unity, the 

 corresponding results will be given by Table III. 



The assumption on which these results are calculated may be called an ' equi- 

 librium theory,' since it takes no account of the motions of the bodies in question, 

 and it assumes a permanent distribution to have been attained, so that the whole 

 of the gas is at a uniform temperature. 



when every allowance is made for the artificial character of the assumptions it 

 is still highly unreasonable to suppose that the Moon could have an atmosphere so 

 far in excess of that required by the equilibrium theory that its presence could be 

 detected even by the most careful observations ; and a very few molecules of 

 oxygen and nitrogen flying about in interstellar or interplanetary space would 

 represent a number far in excess of that required by the equilibrium theory, and 

 would therefore tend to augment the total mass of the Earth's atmosphere. 



If we try to compare the atmospheres of different planets, such as the Earth 

 and Mars, the ' equilibrium theory ' breaks down completely, as is only natural 

 when we remember how rarely a single molecule leaves the atmosphere of either 

 planet. 



It is diff'erent in the case of two bodies so near each other as the Earth and 

 Moon. Among the molecules of gas which at any time might find themselves in 

 the neighbourhood of the Moon and Earth the greater number would be drawn in 

 by the more attractive body, and the Moon would not, therefore, be likely to 

 obtain an atmosphere like that surrounding the Earth. 



At no period has it possessed an atmosphere of oxygen and nitrogen com- 

 parable in density with that of the Earth. A decrease of density in a planet's- 

 atmosphere could only take place by the condensation in liquid form of vapours 

 present in it, not by matter leaving the planet. 



Thus the kinetic theory of gases is capable of accounting for absence of air 

 from the Moon without making any assumptions contradictory to the nebular 

 hypothesis. 



8. On Orinding and Polishing. By Lord Rayleigh, Sec.B.S. 



9. Simple Apparatus for Observing and Photographing Interference 

 and Liffraction Phenomena. By W. B. Croft, M.A . 



A wooden screen ] 6 inches high and 9 inches broad has an opening at a height 

 of 10 inches which will take a spectroscope slit or a thin metal plate with a pin- 

 hole : a convex lens focusses sunlight or limelight on the small aperture ; a lamp, 

 however, gives sufficient light for the main effects without the finer detail. At 

 about 2 feet distance an a or b Huygens' microscope eyepiece is adjusted so that 

 its field is evenly covered with the light ; about 6 inches in front of this is the 

 holder for the diffraction-objects — a stiif-jointed arm about 3 inches long is a 

 convenient adjustment for height. Various things are fixed on 3-inch squares of 

 wood which have a central hole J inch square ; a slot in the middle of one side of 

 the wood goes over a screw at the end of the jointed arm ; a nut clamps it, but 

 allows movement in a vertical plane. The chief simple objects are : Single edge, 

 square corner, double edge, bi-prism, inclined mirrors, needle-eyes, needle-points, 

 needles of various thicknesses, needle with opaque slip on one side, needle with mica 

 slip on one side, perforated zinc, wire gauze, shot cemented on glass for Arago's 

 bright spot at the centre of the shadow of a circular screen, holes of gi-aduated sizes 

 in a metal plate. 



If the eyepiece is passed through a collar which will fix on the front of a 

 carnera in the place of the ordinary lens, an image is made on the groimd glass 

 which can be photographed. The rays emerge parallel and the image varies in 

 size, but remains in focus for all positions of the eyepiece and ground glass. 



In illustration of the two modes of observing these pheiiomtna the author drew 

 attention to an old set of diffraction objects, consisting of lilty-nine small geo- 



