538 



NATURE 



[October 8, 1891 



ice is retreating, a mass of it may be insulated ; as 

 this melts, the superincumbent material tends to slip 

 towards the edges, and thus to form a ring of debris, by 

 which, after the ice has disappeared, a hollow is inclosed. 

 Dr. Wright also adopts the opinion, maintained by Prof. 

 Claypole, the late Prof. H. C. Lewis, and others, that one 

 effect of the advance of this great mass of ice was to 

 obstruct the flow of all rivers which take a northerly 

 course, and thus to convert their valleys into lakes. 



But into a discussion of this interesting question, and 

 of the cause of the glacial epoch, to which a considerable 

 space is devoted, we must not now enter. We must also 

 pass over the questions relating to the date of the glacial 

 epoch and its relation to the first 'appearance of men, 

 merely stating that Dr. Wright inclines to regatd the 

 latter as pre-glacial, but the former as less remote than 

 is generally supposed. It must suffice to say that he 

 appears to be a careful observer, and generally a cautious 

 reasoner, though slightly too prone to quote the remarks 

 of others without due criticism ; so that, on the whole, his 

 book presents us with a good summary of the results of 

 investigations into the glacial geology of North America, 

 and will be valuable for purposes of reference on this side 

 of the Atlantic. T. G. Bonney. 



THE TOTAL REFLECTOMETER AND THE 

 REFRACTOMETER FOR CHEMISTS. 

 Das Totalreflectometer unci das Refractometer fur 

 Chemiker, ihre Veriuendung in der Krystalloptik und 

 zur Untersuchung der Lichtbrechung von Fliissig- 

 keiten. Von Dr. C. Pulfrich, Privatdocenten an der 

 Universitat Bonn, und Assistenten des physikalischen 

 Instituts. With 4 Lithographic Plates and 45 Figures 

 in the Text. (Leipzig : W. Engelmann, 1890.) 



THIS book contains an exhaustive account of one of 

 the latest devices in physical optics for investigat- 

 ing the refractive power of uniaxial and biaxial crystals. 

 The idea of making use of the principle of total reflection 

 for this purpose is not new. Wollaston, at the beginning 

 of the century, brought forward a method in which the 

 crystal plate under examination was attached to a glass 

 prism ; but, owing to the experimental difficulties involved 

 in this process, it met with little practical application. 

 The instrument constructed by Kohlrausch in 1878, in 

 which the crystal plate was immersed in a strongly re- 

 fractive liquid, was a distinct advance, and has been 

 much used. Within the last ten years, also, Wollaston's 

 apparatus has been considerably improved by Fussner 

 aftd Liebisch. Both these instruments, however, have 

 still many inconveniences, and it is the claim of the 

 author that the method which he has devised, and which 

 forms the subject of the present work, is free from these. 

 To give some idea of this method, without entering into 

 practical details, it will be sufficient to state that it consists 

 essentiallyin the replacement of the prismof the Wollaston 

 instrument by a glass cylinder, to the upper plane surface 

 of which the crystal plate is attached. The cylinder can 

 be rotated about its long axis, so that the refractive phe- 

 nomena in all azimuths can be observed. This is the 

 distinguishing feature which forms the chief advantage of 

 the new method. Thus, by illuminating the crystal plate 

 NO. I 145, VOL. 44] 



from the side at grazing incidence, and slowly rotating 

 the cylinder, the whole extent of the Hmiting curves of 

 total reflection comes under observation. By a special 

 method of illumination from all sides the limiting curves 

 maybe received on a screen beneath the cylinder and made 

 visible to a number of observers ; eg. in the case of a 

 uniaxial crystal the appearance on the screen will be the 

 sectional curves of the wave-surface, a circle and an 

 eUipse corresponding to the ordinary and extraordinary 

 rays. 



The method was first suggested by the author four 

 years ago. The object of the present work is to give a 

 complete account of the series of measurements and ob- 

 servations which have been made with the instrument 

 since that time with a view to testing its usefulness and 

 trustworthiness. After some preliminary observations on 

 the theoretical principles involved in the method of total 

 reflection, the author gives a detailed description of the 

 construction of the new instrument and the methods of 

 observation by which it is possible in a single crystal sec- 

 tion to ascertain the position of the axes of elasticity, to 

 measure the optic axial angleTor different colours, and to 

 determine the principal refractive indices. Of special 

 interest is the section on the appearances in the direction 

 of the optic axes of biaxial crystals. Observations madeon a 

 plate of asparagine, cut parallel to the optic axial plane,, 

 showed distinctly the effects due to the internal and 

 external conical refraction, thus supplementing Lloyd's 

 experiments in demonstrating the general correctness of 

 the Fresnel wave-surface. The last section of the book 

 deals with the refraction of liquids, and contains a descrip- 

 tion of the refractometer for chemists, which is a simpli- 

 fied form of the total reflectometer, in which a prism 

 I replaces the cylinder. Altogether, a perusal of the work 

 leaves the impression that the invention of this ingenious 

 and yet comparatively simple method for investigating 

 the refractive power of doubly refractive media marks a 

 decided advance in physical science ; and the author 

 appears to have quite substantiated his claim to have 

 made the total reflection method, which has long been 

 recognized as theoretically the most promising, also a 

 thoroughly practical one. G. T. P. 



A WEATHER RECORD 

 OF THE FOURTEENTH CENTURY. 

 Const deraciones temperiei pro 7 annis, per Magistrem 

 Wilhelmum Merle, socium dornus de Merton. Repro- 

 duced and Translated under the supervision of G. J.. 

 Symons, F.R.S. (London: Edward Stanford, 1 891.) 

 T N January 1337, barely forty-five years after the death 

 -L of Roger Bacon, and ten years after the accessiorb 

 of King Edward the Third, William Merle, a Fellow of 

 Merton College, and Rector of Driby, in Lincolnshire,, 

 commenced a journal of the current weather as expe- 

 rienced partly at his rectory " in Lyndesay, near the 

 north-east coast," and partly at Oxford. This journal he 

 continued month by month for seven years, or up to 

 three years before his death, the notices of the last four 

 years being considerably amplified over the earlier entries ; 

 and the original manuscript, still preserved in the Bod- 

 leian Library, has now, thanks to the initiation of Mr. 



