May 4, 1916] 



NATURE 



199 



elusion that Buckland's stems from Portland, on 

 which the genus Cycadeoidea was founded, bore 

 no lateral fertile shoots like those characteristic 

 of Bennettites, as defined by Dr. Stopes, is not in 

 accordance with a statement made by Buckland 

 in a memoir which appears to have been over- 

 looked. In 1912 Dr. Stop>es published an account 

 of some Angiospermous stems from British Aptian 

 strata, and in the present volume some additional 

 types are described. Impressions which are almost 

 certainly those of Dicotyledonous leaves have been 

 recorded from rocks slightly older than the 

 Lower Greensand, but the specimens described by 

 Dr. Stopes are the oldest known examples of 

 petrified Angiospermous wood. The anatomical 

 characters are carefully analysed and no pains 

 have been spared to compare the fossils with 

 recent forms. As the author points out, the 

 Angiosp>ermous wood so far discovered exhibits 

 no features which can be regarded as primitive, 

 and it is clear that the evolution of the present 

 dominant class had already reached an advanced 

 stage. 



Dr. Marie Stopes has successfully accomplished 

 a laborious and difficult piece of work : the well- 

 illustrated volume is a contribution of permanent 

 value to British Palaeobotany. 



A. C. Seward. 



A NEW TEXT-BOOK OF OPTICS. 

 A Treatise on Light. By Dr. R. A. Houstoun. 

 Pp. xi + 478. (London: Longmans, Green and 

 Co., 1915.) Price js. 6d. net. 



13 ECENT years have witnessed the production 



-'•^ of several good treatises on optics in the 



English language, chief amongst them being 



Preston's "Theory of Light," Schuster's "Theorv 



of Optics," R. B. Wood's "Physical Optics," 



Edser's "Light for Students," and J. P. Southall's 



"Principles and Methods of Geometrical Optics," 



to say nothing of more special works, such as 



jTrotter's "Illumination." But Dr. R. A. 



jHoustoun's "Treatise on Light," now before us, 



joccupies a place of its own. It will be welcomed 



jas a manual for classes of a more advanced char- 



lacter than those in which optics is taken merely 



jas a part of a general physics course. The study 



of optics for its own sake, so neglected in most 



jf the universities, would assuredly receive better 



.jattention if optics were handled in the spirit of 



:his book, and with as full an insight into recent 



ievelopments and investigations. It is, indeed, 



ilive with modern information and research ; and, 



numerous passages reveal, it is written by one 



whom optical laboratory Work is familiar, and 



ho directs it to bring out useful and important 



e suits. 



The book is divided into four parts : — (i) geo- 

 etrical optics; (ii) physical optics; (iii) spectro- 

 metry and photometry ; and (iv) the mathematical 

 meory of light. Incidentally, the topic of physio- 

 Sfical optics is interpolated in part iii. The 

 ction on geometrical optics presents an advance 

 many features over the exposition of that sub- 

 NO. 2427, VOL. 97] 



ject in most text-books, its treatment of thick 

 lenses, of lens combinations, and of aberrations 

 being, on the whole, extremely satisfactory. In 

 few points only does the author give the reviewer 

 occasion to grumble. One of these is his awk- 

 ward convention as to the signs plus and minus, 

 which do not here signify measurement to the 

 right and left, respectively, from any fixed zero 

 or origin. Another is the inconvenient practice 

 of treating all rays as travelling from the right to 

 the left, instead of the more usual left to right. 

 Nowhere does the author give the definition of 

 the metric unit of power of lenses, the dioptrie, 

 though it was adopted internationally in 1875. 

 The only mention of it — and he spells it diopter — 

 is in the brief passage on defects of vision. He 

 builds up the theory of thick lenses quite logically 

 from Helmholtz's tangent law. His brief direc- 

 tions as to the measurement of focal lengths on 

 pp. 75 and 76 are very good. Most unfortu- 

 nately, he uses the Greek letter \ on p. 300, not 

 to denote wave-length, but to signify a coefficient 

 of absorption ; and, in defiance of modern practice, 

 he employs the symbol v, not to denote the anti- 

 dispersion coefficient, but to signify its reciprocal. 

 On p. 65 all that the author has to say on the 

 residual chromatic aberration known as " second- 

 ary spectrum " is that " it can be diminished con- 

 siderably by using- some of the new glasses made 

 in Jena. They appear, however, to offer difficul- 

 ties in manufacture and- to be not very durable." 

 This is scarcely fair to the achievements of Abbe 

 and Schott; for, though their phosphate crown 

 glasses have not proved permanent, their success 

 in producing pairs of crowns and flints that will 

 eliminate secondary dispersion, and in introducing 

 the really valuable novelty of baryta crowns, 

 should be frankly acknowledged. The advantage 

 of using for a lens a glass with a higher index 

 of refraction, as stated on p. 59, diminishes the 

 spherical aberration considerably ; and the baryta 

 crowns give precisely this advantage over the 

 other kinds, while requiring relatively less com- 

 pensation by means of correcting lenses of flint. 

 The author's remarks on the resolving powers of 

 microscopes, telescof>es, spectroscopes, and 

 diffraction gratings are distinctly good. It is a 

 curious point that the ordinary method of describ- 

 ing the working aperture of a lens, so familiar 

 to photographers, as a fraction of the focal length, 

 j is only mentioned in this work in connection with 

 I the Fery spectrograph and the Rowland grating. 

 I Another curiosity in arrangement is the inclusion 

 I of the subject of persistence of vision in the sec- 

 tion headed "Optical Lantern." 



Amongst the outstanding excellences of the 

 ! work we may praise the chapters that deal with 

 \ interferometers and spectrographs. The two chap- 

 I ters on spectroscopy — the earlier and later spectro- 

 scopic work being separated — are very good. 

 The author seems to labour under the erroneous 

 impression, however, that Newton used only a cir- 

 cular aperture and not a slit. There is a cryptic 

 sentence on p. 238, that the dispersion, as speci- 

 I fied by dd/d\, "is easily found experimentally to 



