CH. II] SUMMARY FOR ALTERNATING CURRENT LANTERNS 77 



7. Focus the screen image 

 with special care when using 

 alternating current lest the two 

 sources of light produce a doub- 

 ling of the screen image. 



8. Use opera-glasses, if neces- 

 sary, for focusing sharply a 

 distant screen image (38). 



9. Look out for shadows on 

 the screen. Center carefully 

 and remove all causes for shad- 

 ows (83-93). 



7. Do not forget the greater 

 need for accurate focusing with 

 an alternating current lamp, on 

 account of the double source of 

 light. 



8. Do not forget the advan- 

 tage in using opera-glasses for 

 focusing if the screen distance 

 is great. 



9. Do not permit any defect 

 in the management of the lan- 

 tern, suspended strings, etc., to 

 give shadows on the screen. 



10. Study the "Troubles" in 

 116-118, and 62-98. 



10. Do not neglect any of the 

 causes for "Troubles." 



