NOTICES OF NEW BOOKS. ' 409 



■ 



The method suggested on page 23 for examining opaque materials 

 by reflected light does not altogether commend itself to the reviewer. 

 Eeflecting light by means of a white card on to the specimens would 

 only be feasible when using low-power objectives, and, as the authors 

 imply, the loss of light is considerable. The present writer has obtained 

 good results by tilting the stage to a high angle of inclination, bringing 

 the illuminant behind the microscope (i.e. immediately in front of the 

 observer) and focussing the light on to the slide by means of a bull's-eye 

 condenser. The direction of the incident light being nearly parallel to 

 the line of sight in the microscope, the loss of light is comparatively 

 small. 



The book will assuredly strike the reader as the production of 

 petrologists who have themselves grappled with the practical problems 

 of this branch of microscopy, and are therefore well qualified to act as 

 guides to others who may be experiencing the inevitable difficulties in the 

 preparation and examination of rock-sections. F. I. G. R. 



Simplified Method of Tracing Rays thirough any Optical System. 

 By Ludwik Silberstein, Ph.D. London : Longmans, Green & Co. 

 Price 5s. net. 



The purpose of this small volume is to indicate a method of treating 

 the geometrical optics of any system, particularly that part of a com- 

 puter's work known as "ray-tracing." It is not of special interest to 

 microscopists, unless an interest is taken in general optical problems. 

 The treatment is mathematical, and assumes a knowledge, elementary it 

 is true, of vector algebra. The book has been produced at the insti- 

 gation of Messrs. Adam Hilger, Ltd. It is satisfactory to know that 

 an optical firm in this country, of high repute, is interested in such a 

 publication. J. E. B. 



Elements of the Electro-magnetic Theory of Light. Ludwik 

 Silberstein, Ph.D., Lecturer in Natural Philosophy in the 

 University of Rome. London : Longmans, Green & Co. 1918. 

 Price 3s. Qd. net. 



This little volume, the object of which is to present the essentials of 

 the electro-magnetic theory of light, was rewritten at the instance of 

 Messrs. Adam Hilger, Ltd., from the author's Polish treatise on Elec- 

 tricity and Magnetism. 



The language adopted is mainly vectorial, so that while the treat- 

 ment of the subject is of necessity mathematical, it does not entail a 

 knowledge of the higher mathematics for its intelligent appreciation. 

 The author accurately describes it as an easy and complete presentation 

 of the fundamental part of Maxwell's theory of light, and as such may 

 be commended to those microscopists whose interests are mainly directed 

 to the optical side of then- subject. J. E. B. 



