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OUR PHYSICAL WORLD 



sunny day will therefore be much shorter with any given dia- 

 phragm and plate than on a dull, cloudy day. One can learn 

 by experience to judge the length of exposure under varying 

 light conditions with different- sized diaphragms and different 

 plates, but it will be at the expense of spoiling many plates. 



It is advisable, therefore, to purchase and use an exposure 

 meter in order to save both time and material. Cheap ones 

 can be obtained which will indicate the exposure for any sized 



diaphragm under most condi- 

 tions, such as time of day, 

 season, cloudiness of the sky, 

 nature of the object to be 

 photographed. They are not 

 as satisfactory under excep- 

 tional conditions, such as 

 photographing in deep woods 

 or indoors, as are the types 

 in which one exposes a strip 

 of sensitive paper to find the 

 light intensity. The method 

 of operation of one such may 

 be given as typical. The ex- 

 posure meter can be opened 

 as one would take off the 

 back of a watch, and a strip 

 or disk of sensitive paper be 

 laid in, after which the back is closed again. The front of such 

 an exposure meter is shown in Figure 158. The little opening 

 through which light gains admission to the sensitive paper 

 is kept covered by a piece of ruby glass until one is ready to 

 use the instrument. At one side of this opening is a sample 

 of dark paper of fixed tint. One holds the exposure meter in 

 the moderate shadows of the object to be taken, then turns 

 aside the colored glass so a bit of the sensitive paper is exposed, 

 and notes in seconds the time required for it to darken sufficiently 



FIG. 158. An exposure meter 



