Smithsonian Report, 1906. — Laurie. 



Plate 



Mierophotograph of scratch through cheap resinous 

 varnish, showing oonchoidal fracture. 



1. Varnish testing machine. 



3. Mierophotograph of crack in picture, showing 

 cracked upper layer (dark), and uninjured 



lower layer (light). 



4. Mierophotograph of section through cracked 

 picture. 



Varnish Testing and Microphotography of Cracked Pictures. 



