vi OPTICAL PROJECTION 



other style would have been ; and that the frequent use of 

 italics is not for emphasis in the usual sense, but chiefly 

 as the easiest way of marking points it is desirable to 

 distinguish with some clearness. 



I have to thank Messrs. Macmillan & Co. for permis- 

 sion to use the greater part of the illustrations to my 

 previous work, entitled ' Light : a Course of Experimental 

 Optics,' and also the publishers of Professor Weinhold's 

 ' Physikalische Demonstrationen ' (Leipzig) and Dr. 

 Stein's ' Die Optische Projektionskunst ' (Halle) for many 

 illustrations from those works. Most of the others are 

 original ; but a few have also been taken from Ganot's 

 ' Elements of Physics,' Prof. Forbes's ' Lectures on 

 Electricity,' some articles by Mr. G. M. Hopkins in the 

 ' Scientific American,' and Prof. Dolbear's * Art of Pro- 

 jecting' (Boston). The very few experiments taken or 

 adapted from the latter work, are owing to the fact that 

 it is written with especial reference to projections with 

 the heliostat, which is almost useless in this country. 

 Many such arrangements I have found unsuitable for 

 ordinary lantern use ; but where sunlight is available, 

 Prof. Dolbear's treatise may be consulted with advan- 

 tage. 



My grateful acknowledgments are finally due to my 

 old friend and correspondent, the Eev. P. E. Sleeman, 

 for reading the proofs of the last twelve experimental 

 chapters, during which process several suggestions and 

 additions of value have also found their way into those 

 pages. 

 LONDON : October 31, 1890. 



