November 4, 19 15] 



NATURE 



253 



Tycho made a g"Ood determination of the leng-th 

 of the tropical year, his error being only i second. 

 He also obtained a good value of the constant 

 of precession, by comparison of his results with 

 those of Hipparchus and Ptolemy ; he was able to 

 show that the large " trepidations " or fluctuations 

 in this motion, that were previously believed to 

 exist, were wholly illusory. 



Tables of the sun are given in the volume, the 

 data extending to the year 1800. The annual 

 advance of the perigee is taken as 45", the true 

 value being 61". A short table of astronomical 

 refraction is given ; Tycho was one of the first 

 to investigate this correction ; his value 34' for 

 khe horizontal refraction agrees with modern au- 

 [thorities, but he gives 5" instead of 57" for the 

 ralue at 45° altitude. The erroneous assump- 

 tion of 3' for the solar parallax vitiated the 

 ^fractions that were based on solar observations, 

 "ycho's instruments were capable of proving that 

 le true value was much smaller, but it never 

 xurred to him to suspect the accepted value. 

 The section on the moon is specially interesting, 

 for Tycho added important facts to our know- 

 idge of its motion. He was an independent dis- 

 >verer of the "variation," even if we admit the 

 loubtful claim of- Abul Wefa to be the first dis- 

 )verer; in any case his writings were unknown 

 Europe until long after Tycho's time. Tycho 

 Iso discovered the important "annual equation," 

 md drew attention to the " reduction " arising 

 from the inclination of the moon's path to the 

 fecliptic. The mean values for the parallax and 

 'semidiameter adopted by Tycho, 60' 51" and 17^0'' 

 respectivelv. are both too great, and further from 

 the truth than the values 58' 25" and 16' 25" pre- 

 lyiously adopted by Copernicus. The latter, how- 

 jver, adopted an eccentricity double the true value, 

 ind Tycho greatly improved this element. Lunar 

 ibles are given in the volume, and their use 

 Uustrated by calculations of the details of the 

 ilipses of the sun and moon that occurred near 

 le end of the year 1601. 



The chapter on the determination of the co- 

 jrdinates of the fixed stars opens with an account 

 »f the methods previously employed for connecting 

 leir places with that of the sun. The moon had 

 generally been employed as an intermediary, either 

 rhen visible in daylight, or when eclipsed, it being 

 jbviously practicable to connect the longitude of 

 le shadow on the moon with that of the sun. 

 'ycho used Venus as an intermediary ; he found 

 |t possible to observe it in daylight when at its 

 greatest brilliancy; the fact of its parallax, semi- 

 liameter, and motion being much smaller than 

 lose of the moon were great advantages. His 

 star observations are astonishingly good, con- 

 NO. 2d.OI. VOL. q6] 



sidering that no optical aid was available in his 

 day ; for example, fifteen values of the right ascen- 

 sion of Alpha Arietis, made between 1582 and 

 1587, and reduced to the end of 1585, range from 

 26° o' 4" to 26° o' 44", the mean value being 

 26? o' 29", only 19'' less than the value calculated 

 from modern data. Such work compares favour- 

 ably with some that was done after the invention 

 of the telescope, and it must be remembered that 

 Tycho was ignorant of the existence of nutation 

 and aberration. 



He selected Alpha Arietis as the "Lapis Angu- 

 laris " of his celestial structure, its proximity to 

 the vernal equinox rendering it convenient for this 

 purpose. A short list of fundamental stars, 

 mainly in the Zodiac, have their places determined 

 with special care. Then is given a less accurate 

 catalogue of the longitudes and latitudes of about 

 800 stars for the epoch 1600. This must have 

 been a great boon to his contemporaries, as the 

 existing star catalogues were very inaccurate. A 

 hundred bright stars have their right ascension 

 and declination given for 1600 and 1700. 



The reprint follows the original closely in type 

 and pagination, the illustrations being reproduced 

 in facsimile. They include diagrams of the in- 

 struments, which claim the admiration of astro- 

 nomers, as the source of the observations from 

 which Kepler deduced the laws of planetary 

 motion. 



The work is enriched with some annotations by 

 Dr. Dreyer, directing attention to slips and inac- 

 curacies in Tycho's text, or giving further infor- 

 mation on various matters of interest. 



Andrew C. D. Crommelin. 



PLANT-BREEDING. 

 Plant-breeding. By Prof. L. H. Bailey. New 

 edition, revised by Arthur W. Gilbert. Pp. 

 xviii + 474. (New York : The Macmillan Co. ; 

 London: Macmillan and Co., Ltd., 1915.) 

 Price 8s. 6d. net. 



THE new edition of Prof. Bailey's text-book 

 of plant-breeding contains a good deal of 

 fresh matter. The work first appeared in the time 

 when no order had been perceived in the pheno- 

 mena of variation or heredity, and chapters by 

 Dr. Gilbert are introduced dealing with the re- 

 cent discoveries. These sections of the book are 

 well done so far as they go, but though various 

 parts have been rewritten, signs of irregular 

 growth remain, as is, indeed, in such cases un- 

 avoidable. 



In his preface Prof. Bailey expresses with some 

 frankness a regret that with the new knowledge 

 the old simplicity of ignorance has been troubled. 



