viii OPTICAL PROJECTION 



It is under these sad circumstances that I have 

 taken up the pen in order that, as nearly as possible, 

 the fourth edition should be presented to the public as 

 it would have been had the Author been spared to 

 revise it. 



So far as possible I have preferred to leave my 

 father's original work untouched, a few paragraphs 

 slightly altered and a footnote here and there comprise 

 all the alterations that have been made, and the bulk 

 of the work has consisted in writing a new and fairly 

 voluminous Appendix describing many of the more 

 modern optical instruments. In doing so I have 

 followed, so far as possible, my father's plan of including 

 no instruments or experiments that I cannot speak of 

 from practical experience. 



With hardly an exception every paragraph in the 

 new Appendix has been written from my own personal 

 knowledge. In a work of this class, dealing as it does 

 with the easiest and most convenient way of handling 

 apparatus, to do otherwise would be dangerous and 

 unwise, and this must be my apology if omission has 

 been made of any new appliances which should have 

 been included. 



KUSSELL S. WEIGHT. 



June 1906. 



