PREFACE TO THE SECOND EDITION. 



SINCE the first publication of this book the author has received 

 so many commendatory letters from many parts of the world that 

 he is fully persuaded that the book met a real want ; and a sun- 

 beam is now made useful in school work and in the study of 

 phenomena in many places where no substitute is practicable. In 

 preparing a new edition, some things have been added which it is 

 hoped will make the book still more useful to such as consult it. 

 Two things may be specially mentioned here : the electric lamps 

 and lights for projection purposes, and the production and phe- 

 nomena of vortex rings. Of the former there is pointed out 

 what is at present practicable, and of the latter it may be said that 

 the vortex-ring theory of the constitution of matter has so much 

 philosophical as well as scientific importance, and the phenomena 

 presented by vortex rings are so curious and unexpected, that the 

 author has felt warranted in presenting what he believes to be the 

 most complete series at present known, especially as he believes 

 himself to be the discoverer of a considerable number of them. 



Several small treatises on the management and use of lan- 

 terns have been published lately, and may be had on application to 

 almost any of the larger dealers in physical instruments and lan- 

 tern transparencies. It was not thought advisable to add anything 

 on that subject. An excellent manual for experimental work with 

 the lantern has been published by Lewis Wright, which every one 

 interested in such matters should have, especially as many of the 

 experiments described in it can be done as well or better by the 

 use of a beam of sunlight, the use of a beam of sunlight for 

 projection being the peculiar province of this book. 



A. E. DOLBEAR 

 COLLEGE HILL, MASS., 

 Sept. gth, 1887. 



