PHOTOGRAPHY 107 



stick, simple screen (cardboard 3" X 3" standing in slotted 

 block). 



Materials: One candle mounted on a block, four candles 

 mounted on a block. 



a. Place peg as near to screen as possible and place the one 

 candle 20 cm. from the peg. How far away do you have to 

 place the four candles to obtain a shadow as dense as the one 

 candle casts ? Repeat, placing single candle at a distance of 

 30 cm. A slightly darkened room is desirable. 



6. Using this method test the candle power of an incan- 

 descent lamp at home. Also test the candle power of a gas 

 flame and of a kerosene lamp by this means. Make your 

 own photometer. 



79. PHOTOGRAPHY 



The waves near the violet end of the spectrum are the most 

 active in the production of chemical changes, and they are also 

 those which kill the bacteria of disease. Proper ventilation 

 and sunlight can do much toward sanitation. 



The chemical effect which is made use of to the greatest 

 extent in the arts is that of sunlight upon the combinations of 

 silver and other elements. Silver chloride and silver bromide 

 turn dark in the sunlight, due to the separation of metallic 

 silver from its compound. After a silver salt has been exposed 

 to light, it can be assisted in the separation of metallic silver 

 by certain chemicals, called developers. They are known 

 chemically as reducing agents. 



There are other photographic effects of light which are not 

 commonly thought of under this title. The tanning of the 

 skin is just as truly a chemical change, and a photographic 

 result, as is the breaking down of a silver compound. 



The details of photography, although very interesting, and 



