4- PREFACE. 



Dr. E. L. Maddox that I need not particularize any late favors. 

 I thank certain opticians for blocks which appear in these 

 pages, notably Mr. Powell, Mr. Swift, Mr. Beck, Mr. Baker, 

 and Mr. Newton. 



Apology is perhaps due for over frequent reference to a 

 book in the writing of which I took part. It seemed better 

 to refer even to my own writings than to waste space in the 



present book. 



Andrew Pringle. 



