172 



NA TURE 



[June 21, 1906 



instead of uniting- several as subgenera under one 

 main genus, here again differing from Engiisli 

 authorities. A good instance is afforded by a 

 comparison of the species united under Weisia by 

 Dixon with the same species that are referred by 

 Roth to seven different genera ; and two of these are 

 quite separated from the others, as they fall under 

 the Cleistocarpi. In his treatment of the Hypnacese, 

 to which family one naturally turns, Roth also differs 

 considerably from Schimper. Dixon collects ten of 

 Schimper's genera under his genus Hypnum, but speci- 

 fies five of them as subgenera. Roth has twelve 

 equivalent genera, but again four genera are placed 

 in a different family, and unnecessarily, although 

 not without reason, the genus or subgenus Harpidium 

 is changed to Drepanocladus. This is one of the 

 names which, it is hoped, will be sustained when the 

 nomenclature of the cryptogams is discussed at the 

 next International Botanical Congress. 



The points of difference between this work and 

 Dixon and Braithwaite's books are so numerous that 

 British moss-workers will refer to the " Europaischen 

 Laubmoose " for contrast rather than for comparison. 

 As a practical handbook for naming mosses the work 

 deserves much commendation, and bears ample evid- 

 ence that the writer has definite opinions to express. 

 It is a great pity that analytical tables for distinguish- 

 ing genera are not given, but the descriptions are 

 good, and important characters are thrown into bolder 

 type. References to the most recent discoveries of 

 new localities and new varieties will be found. The 

 illustrations are very numerous, but poor in execution. 



PHOTOGRAPHY IN SURVEYING OPERA- 

 TIONS. 

 All Elementary Treatise on Pliototopographic 

 Methods and Instruments. By J. A. Flemer. 

 Pp. xix + 438. (New York: John Wiley and Sons; 

 London : Chapman and Hall, Ltd., igo6.) Price 

 21s. net. 



THE assistance that photography can render in 

 the laborious work connected with topo- 

 graphical surveys has been repeatedly insisted upon, 

 and the recognition of the fact is being displayed 

 in the construction of a class of instruments admirably 

 adapted for use in the field. With the more con- 

 venient instruments that experience will suggest, and 

 with the shortened methods that familiarity will 

 supply, the employment of photography is likely to 

 be still more general, though doubtless it will have 

 to contend against a certain amount of prejudice in 

 favour of older methods. 



Mr. Flemer's book is intended to overcome these 

 prejudices and to determine the exact field which the 

 camera can usefully occupy in surveying operations. 

 The method has its limitations. The accuracy of a 

 map constructed from panoramic views must evidently 

 depend upon the precision with which objects can be 

 identified and measured on a photograph or its en- 

 largement. The use of telephoto-lenses or long- 

 focused cameras would increase the accuracy, but at 

 NO. 191 2, VOL. 74] 



the expense of convenience. More numerous photo- 

 graphs taken at shorter distances would also have 

 the same effect, but then the labour of collecting and 

 reducing the material would approach that due to 

 the ordinary methods. It is the object of such a book 

 as this to show that photography has distinct ad- 

 vantages peculiar to itself. But in many directions 

 in which extreme accuracy is unnecessary, perspective 

 views can be of essential service. A series of pano- 

 ramic pictures showing the alteration in the face of 

 the country due to volcanic eruptions, or the re- 

 current changes in sand dunes caused by winds blow- 

 ing from certain directions at regular intervals, seem 

 to be peculiarly suitable inquiries for photographic 

 methods. Similarly, the changes in glacier form- 

 ation and the determination of their motion, alter- 

 ations in coast-line due to erosion, or the location of 

 rocks and buoys would suggest other applications for 

 the process. In wars and manoeuvres, either with 

 or without the use of balloons, the process has a 

 large field of application. 



Since the translation of a perspective view or com- 

 bination of views into maps possessing considerable 

 accuracy of detail is likely to concern many other 

 professions than that of a surveyor, properly so called, 

 it is most desirable to have a treatise in which is 

 set out clearly the methods of construction and of 

 the principles underlying the practice of the process. 

 This want Mr. Flemer's book is intended to supply. 

 That the author is competent to deal with the subject 

 practically we entertain no doubt, but whether he 

 has been successful in conveying his knowledge to 

 others it is very difficult to judge. Surveying is 

 technical work that can hardly be learnt without 

 actual experiment in the field under the eye of a 

 trained teacher. What amount of preliminary in- 

 formation is a pupil supposed to have before tackling 

 the problems the author introduces? Certainly one 

 ought to be accustomed to the use of the plane table 

 and the time-honoured methods of procedure before 

 addressing himstlf to the study of perspective views. 

 The method should be regarded, not as a substitute, 

 but as an addition, to the recognised processes. 



Mr. Flemer's book consists practically of three parts. 

 After a short preliminary historic review to show 

 how the principles have been welcomed in various 

 countries, the author discusses the phototopographic 

 methods that various authorities have recommended. 

 The fact that we have so many varieties of detail 

 shows that the subject has not 3'et taken that me- 

 chanical, stereotyped form which it may be expected 

 to assume when fully developed. The second part 

 opens with a chapter on lenses, which seems hardly 

 necessary to introduce the description of the many 

 photogrammetric instruments now in use. This latter 

 is a really valuable and excellent section. Finally, we 

 have the details of the photographic operations, in- 

 cluding the development of the plates and prints. 

 There is not much that is new that can be said here, 

 and the treatment of pinhole photography which is 

 naturally connected with this subject is unfortunately 

 discussed in another place. W. E. P. 



