August 15, 1901] 



NA TURB 



585 



This star has been under observation for nearly twelve months, 

 and some 200 measures obtained. The period adopted is 



6d. loh. I9'6m., 

 and the light-curve based on this value is jjiven, with an en- 

 lari^ed diagram of the part near minimum. The curve is almost 

 identical in form with that of S Velorum. Other details are as 

 follows : — 



Limits of variation are g'l and lO'S magnitude. 



Duration of increasing or decreasing phase = 4h. 15m. 



Stationary period at minimum =8h. 30m. 



The system thus apparently consists of two bodies, one of which 

 is Ihrec times the diameter of the other. The smaller star is 

 nearly twice as bright as the larger one, and the distance be- 

 tween their circumferences is about two-thirds of the radius of 

 the orbit. The density of the system is probably not more than 

 one-si.xth that of the sun. 



V Puppis. 



R.A.= 7h. 55m. 22s. l,,Qoo.o> 

 Decl. = - 48° 58' -4 J '.'900 0). 



This star differs from the preceding one in that it consists of 

 two bodies of about equal size and brightness. The mean 

 period, as deduced from the light variation, is 



id. loh. 54m. 267s. 

 The light-curve of this star is strikingly similar to that of U 

 Pegasi, showing double and niuqiial minima, and double and 

 equal maxinta. 



Prof. Pickering, however, from spectroscopic determinations, 

 deduces a period of 



3d. 2h. 46m. 



From the peculiarity of there being no stationary period at 

 either maximum. Dr. Roberts infers that the two component 

 stars revolve around each other in actual coulacl. Under such 

 conditions, both bodies would most prob.ably undergo distortion. 

 The value derived for the density of V Puppis is o o? that of 

 the sun, the orbit being circular. 



POLISH> 

 ^j'HE lecture commenced with a de.scription of a home-made 

 spectroscope of considerable power. The lens, a plano- 

 convex of 6 inches aperture and 22 feet focus, received the rays 

 from the slit, and finally returned them to a pure spectrum formed 

 in the neighbourhood. The skeleton of the prism was of lead ; 

 the faces, inclined at 7o^> were of thick plate-glass cemented 

 with glue and treacle. It was charged with bisulphide of carbon, 

 of which the free surface (of small area) was raised above the 

 operative part of the fluid. The prism was traversed twice, 

 and the effective thickness was 5^ inches, so that the resolving 

 power corresponded to 11 inches, or 28 cm., of CS.,. The 

 ■ liquid was stirred by a perforated triangular plate, nearly fitting 

 the prism, which could be actuated by means of a thread within 

 reach of the observer. The reflector was a flat, chemically 

 silvered in front. 



So far as eye observations were concerned, the performance 

 was satisfactory, falling but little short of theoretical perfection. 

 The stirrer needed to be in almost constant operation, the 

 definition usually beginning to fail within about twenty seconds 

 after stopping the stirrer. But although the stirrer was quite 

 successful in maintaining uniformity of temperature as regards 

 space, i.e. throughout the dispersing fluid, the temperature was 

 usually somewhat rapidly variable with time, so that photo- 

 graphs requiring more than a few seconds of exposure showed 

 inferiority. In this respect a grating is more manageable. 



The lens and the faces of the prism were ground and polished 

 (in 1S93) upon a machine kindly presented by Dr. Common. 

 The flat surfaces were tested with a spherometer, in which a 

 movement of the central screw through l/iooooo inch could 

 usually be detected by the touch. The external surfaces of the 

 prism faces were the only ones requiring accurate flatness. In 

 polishing, the operation was not carried as far as would be 

 expected of a professional optician. A few residual pittings, 

 although they spoil the appearance of a surface, do not interfere 

 with its performance, at least for many purposes. 



In the process of grinding together two glass surfaces, the 



1 A discourse delivered at the Royal Institution on Friday, March 20, by 

 the Right Hon. Lord Rayleigh, F.R.S. 



particles of emery, even the finest, appear to act by pitting the 

 glasses, i.e. by breaking out small fragments. In order to save 

 time and loss of accuracy in the polishing, it is desirable to 

 carry the grinding process as far as possible, using towards the 

 close only the finest emery. The limit in this direction appears 

 to depend upon the tendency of the glasses (6 inches diameter) 

 to seize, when they approach too closely, but with a little care it 

 is easy to attain such a fineness that a candle is seen reflected 

 at an angle of incidence not exceeding 60% measured as usual 

 from the perpendicular. 



The fineness necessary, in order that a surface may reflect and 

 refract regularly without diffusion, viz. in order that it may 

 appear polished, depends upon the wave-length of the light and 

 upon the angle of incidence. At a grazing incidence all sur- 

 faces behave as if polished, and a surface which reflects red 

 light pretty well may fail signally when tested with blue light 

 at the same angle. If we consider incidences not too far re- 

 moved from the perpendicular, the theory of gratings teaches 

 that a regularly corrugated surface behaves as if absolutely plane, 

 provided that the ivave-lengtii of the corrugations is less than 

 the wave-length of the light, and this without regard to the dcplh 

 of the corrugations. Experimental illustraticns, drawn from the 



NO. 1659, VOL. 64] 



sister science of acoustics, were given. The source was a bird-call 

 from which issued vibrations having a wave-length of about I "5 

 cm., and' the percipient was a high-pressure sensitive flame. 

 When the bird-call was turned away, the flame was silent, but 

 it roared vigorously when the vibrations were reflected back 

 upon it from a plate of glass. A second plate, upon which 

 small pebbles had been glued so as to constitute an ideally 

 rough surface, acted nearly as well, and so did a piece of tin 

 plate suitably corrugated. In all these cases the reflection was 

 regular, the flame becoming quiet when the plates w-ere turned 

 out of adjustment through a very small angle. In another 

 method of experimenting the incidence was absolutely perpen- 

 dicular, the flame being exposed to both the incident and the 

 reflected waves. It is known that under these circumstances 

 the flame remains quiescent at the nodes and flares most vigor- 

 ously at the loops. As the reflector is drawn slowly back, the 

 flame passes alternately through the nodes and loops, thus exe- 

 cuting a cycle of changes as the reflector moves through liaif a 

 wave-length. The effects observed were just the same whether 

 the reflector were smooth or covered with pebbles, or whether 

 the corrugated tin plate were substituted. All surfaces were 

 smooth enough in relation to the wave-length of the vibration 

 to give substantially a specular reflection. 



Finely ground surfaces are still too coarse for perpendicular 

 specular reflection of the longest visible waves of light. Here 

 the material may be metal, or glass silvered chemically on the 



