66 



DISCOVERY 



beads on the rings — obviously particles pulled out of 

 the plane of the rings by some external influence. 

 Lowell's later mathematical analysis showed that 

 both the gaps in the ring and the knots were due 

 to the pull of the several satellites. " We find," 'he 

 wrote in 1910, " that Mimas, Enceladus, and Tethsy 

 have periods exactly commensurate with the divisions 

 of the rings ; in other words, these three inner satellites 

 whose action because of proximity is the greatest have 

 fashioned the rings into the three parts we know." 



Obviously the ring-system represents a satellite 

 spoiled in the making, and evidently the rings are 

 analogous to the asteroidal zone in the Solar System. 

 In the case of the asteroids, the proximity of the huge 

 mass of Jupiter evidently prevented the formation of 

 a planet of any considerable size. In the case of the 

 Saturnian rings, no satellite could be formed at so 

 small a distance ; and thus we have what is to all 

 intents and purposes a meteor stream — myriads of 

 minute stones and particles closely packed together 

 which at the enormous distance of nearly eight hundred 

 millions of miles from the terrestrial observer appear 

 as an annulus of " immaterial light." 



BIBLIOGRAPHY 



Clrant : History 0} Physical Astronomy (1862, Baldwin). 

 Proctor : Other Worlds than Ours (1870, Longmans). 

 Clarke: History of Astronomy during the Nineteenth Century 



(4th ed. 1903 ; A. and C. Black). 

 Lowell: Mars (1894, Longmans); The Solar System (1903, 



Houghton Mifflin cS: Co.) ; Mars and its Canals (1906, 



.Macmillan) ; Mars as the Abode of Life (1908, Macmillan); 



Evolution of Worlds (1910, Macmillan). 

 Pickering : Reports on Mars, reprinted from Popular A stronomy, 



1914-21. 

 Phillips: B.A.A. Journal, October 1915: Presidential .address 



to BA.A. 



Immunity in Infectious 

 Diseases 



By A. E. Boycott, M.D., F.R.S. 



Craliam Professor 0/ Pallmlogtj in llie Unii-ersily 0/ London, Director 

 of Pathology in University College Hospital and Medical School 



Everybody knows, as a matter of domestic experience, 

 that children who have had measles or chicken-pox are 

 very unlikely to have a second attack of the same 

 disease, and that, when the puppy is through with its 

 distemper, there is no substantial chance of its having 

 it again. It is familiar, too, that an attack of measles 

 does not protect against chicken-pox, or scarlet fever, 

 or German measles, or, indeed, any infectious disease 

 except measles, and that distemper is no preventive of 

 worms or mange. Most people are also prepared to 

 agree that vaccination is a fairly good insurance against 



small-pox, and that antityphoid inoculation improves 

 one's chance of avoiding typhoid fever. In the former 

 case the vaccine Ivmph which is scratched into the 

 skin causes a mild attack of small-pox as it occurs in 

 the cow ; in the latter instance dead typhoid bacilli 

 are injected under the skin. Here, again, vaccination 

 is no protection against typhoid, nor is antityphoid 

 inoculation of any value in escaping small-pox. It 

 appears, therefore, that an attack of an infectious 

 disease either naturally occurring or artificially induced, 

 or a dose of the organisms which are the cause of the 

 disease, may bring about a definite change in the con- 

 stitution of the body. This resistance, or immunity, 

 holds good only for the particular infection which 

 originates it ; it is, in other words, " specific." It 

 varies in intensity in different individuals and with 

 different diseases, being almost absolute in the case of 

 chicken-pox, relatively slight in that of the common 

 cold. It lasts, too, for varying times, the tendency 

 being always for it to diminish progressively : soon 

 after the 1918-19 influenza epidemic, most people who 

 had had an attack could count on not having a second 

 one ; but recent experience indicates that this resist- 

 ance has now largely faded away. Probably a more 

 important fact to reahseis that immunitj- is not acquired 

 at once. If it were, it would be difficult to imderstand 

 how infectious diseases so often run their characteristic 

 courses, and end in recovery. Generally speaking, resis- 

 tance does not develop till about ten days after the 

 beginning of an attack of infectious disease or the inflic- 

 tion of vaccination. For the substances which are 

 introduced into the economy in antityphoid inoculation 

 do not in themselves directly help the body to resist the 

 tj-phoid bacillus. They are not like the doses of quinine, 

 which, with luck, will protect against malaria ; quinine is 

 itself poisonousto the malarial parasite, and is efficacious 

 directly it has entered the body. These dead bacilli, on 

 the contrary, act by stimulating the body to set its 

 tissues to do things which are injurious to the bacteria : 

 antityphoid inoculation or vaccination or an attack of 

 measles only help the body which is prepared to help 

 itself. And the changes in resistance which these 

 experiences of infectious micro-organisms bring about 

 are simply the changes which practice will cause in any 

 organ of the body. A man who has never been in any 

 relationat all to a disease-producing micro-organism will 

 have a certain amount of ability to resist its first attack 

 on his person : resistance may be small or it may be 

 considerable, but there will always be some, for no 

 infectious disease kills all the people it affects. But, 

 once anybody has successfully resisted an invasion by 

 bacteria, his tissues profit by the experience they have 

 had, and are in a better position to resist a second or 

 subsequent attack, just as any bodily action can be 

 trained and improved by practice. 



