EXPERIMENT 46. PHOTOGRAPHIC PAPER 



To prepare a photographic film of blue print paper: 

 To about 24 cu. cm. of water in a small beaker add 4 gm. 

 of ammonium citrate of iron. In another beaker dissolve 

 4 gm. of potassium ferricyanide, and then in a dark room 

 thoroughly mix the contents of the two beakers in a clean 

 sTiallow dish. 



Lay on a table a piece of common glazed paper and give 

 the paper a coating of the mixture, applying the liquid by 

 means of a bit of absorbent cotton. Hang up the paper 

 to dry, taking care that no light falls upon it. 



When the paper is thoroughly dry, lay a magnet or 

 other convenient object upon it and place the paper and 

 the object in the direct sunlight for a few minutes. Re- 

 move the magnet and wash the paper in hot water ; 

 a clear sharp negative of the object will appear on the 

 paper. 



An ordinary negative on a glass plate can be substituted 

 for the opaque object and in that case the blue printing 

 paper will show a positive. 



EXPERIMENT 47. LENSES 



To find the focal length of a convex lens. 



I. Place a convex lens in front of a distant object, as a 

 church steeple or the chimney of a house, in such a way 

 that it will cast an image of the object upon a sheet of 

 paper. Move the paper until the image on it is distinctly 

 outlined, and carefully measure the distance from the paper 

 to the lens. The rays coming from a distant object are 

 practically parallel, but after refraction through the lens 



