DECEilBEB 4, 1908] 



SCIENCE 



801 



tion plays an important role in high-power 

 work with the microscope no one will deny. 

 This author gives it its true value in micro- 

 scopic vision, connecting all vision together 

 instead of making a violent change some- 

 where. 



In this work the statement is frequently 

 reiterated that the eye of the observer forms 

 a part of the optical apparatus, being in this 

 respect in refreshing contrast to those that 

 ignore the eye in dealing with the microscope. 

 The author takes up every other difficult ques- 

 tion relating to the microscope, as angular and 

 numerical aperture, dark-ground illumination, 

 the production of critical images, the limita- 

 tion of microscopic vision propounded by 

 Helmholtz and others, and each subject is 

 simplified. 



With reference to the Helmholtz theory of 

 the limitation of resolution in microscopic 

 vision, the statement or formula of Helmholtz 

 is cast in a form familiar at present in micro- 

 scopical optics and is shown to be 0.6 A/N.A., 

 that is, the wave-length of the light used multi- 

 plied by 0.6, and this divided by the numerical 

 aperture (N.A.) of the objective will give the 

 limits of visibility. Numerous examples are 

 given showing that this theoretical limit is 

 very close to the actual limit with the best 

 modern microscopes as ordinarily used. The 

 point is not yielded, however, without showing 

 a possible escape from the apparent restric- 

 tions. In dealing with high magnifications 

 he says : 



The three impediments to resolution just re- 

 ferred to are: (a) diffusion, (6) conspicuous anti- 

 point and (c) obfuscation in the eye. 



He shows that all these defects are directly 

 referable to the contraction of the beam inci- 

 dent to high magnifications, and that the only 

 way to escape these limitations is to increase 

 the aperture or, as he puts it, " to open up the 

 terminal beam." He then considers the 

 method of Gordon by which the terminal 

 beam is widely opened by means of a disc of 

 ground glass placed at the level of the dia- 

 phragm in the Huygenian ocular, that is, at 

 the level where the real image is formed. 

 This real image is then observed by a second, 

 low-power microscope. To avoid the obscurity 



given by the grain on the ground glass the 

 latter is rotated. 



It is stated that by this means Mr. Gordon 

 has shown repeatedly before the Royal Micro- 

 scopical Society objects under " magnifying 

 powers of 10,000 diameters and over," the 

 retinal image being free from obfuscation and 

 conspicuous antipoint. To enforce the argu- 

 ment and to show the reader the appearance 

 with and without the opening up of the 

 terminal beam, Plate XVIII., with five photo- 

 graphs, is given, illustrating in the most stri- 

 king manner the points mentioned in the text. 



This book is beautifully printed and its 

 numerous figures really illustrate the text. 

 In a word it is the clearest and most authori- 

 tative exposition of the microscope and its 

 accessories and the interpretation of micro- 

 scopic appearances to be found in any single 

 work. It is sincerely hoped that it will 

 speedily find its way into the hands of teachers 

 and advanced students. 



S. H. G. 



Selectionsprinzip und Prohleme der Art- 



hildung. Ein JSandbuch des Darwinismus. 



Von Dr. LuDwiG Plate. Dritte, sehr 



vermehrte Aufiage, mit 60 Figuren im Text. 



Leipzig, Engelmann. 1908. Pp. viii + 



498. 



That Plate's work on the principle of selec- 

 tion and the problem of the origin of species 

 should in eight years have passed through 

 three editions is welcome evidence that the 

 reading public appreciates a good biological 

 treatise; that it should in the respective edi- 

 tions pass from 153 pages and 247 pages 

 to over 500 pages with illustrations is evidence 

 both of the great recent growth of contribu- 

 tions to the subject and of Plate's enterprise 

 in following them up. 



The principal additions to this last edition 

 consist of a brief review of Darwinism, a dis- 

 cussion of Darwin's and de Vries's views of 

 the role of individual variation; an extension 

 of the section on " Sprungevolution " ; a con- 

 sideration of a new objection to Darwin's prin- 

 ciple of selection, viz., that selection can not 

 be demonstrated in detail; additional con- 

 sideration of the forms of the struggle for 



