171 NATURE-STUDY REVIEW [12:4-April, 1916 



subject can be distinguished as dark areas on the back of the plate, 

 even if the emulsion side becomes entirely black. At this point, 

 the plate should be removed from the developer, washed in water 

 and put into the fixer, which will dissolve out the unused silver 

 salts. It should remain in the fixer twice as long as it takes all the 

 white of the emulsion to disappear, which will be less than 10 

 minutes with a fresh solution. 



Glass or hard rubber-fixing baths with room for a dozen plates 

 can be used when developing several at once. 



When the plate is sufficiently fixed and cleared, it should be 

 washed in running water, or else in frequent changes of water, for 

 at least one-half hour. It is then ready for drying, and may be 

 placed on a special drying rack, or leaned against the wall. When 

 dried, it may be compared with the other plates which have been 

 developed, fixed and washed n a similar way, by holding them to 

 the light and looking through them. A clear sharp "negative" 

 image should be seen. If the negative is opaque, it has been over- 

 exposed; if the image is faint it has been under-exposed. 



Ability to use the aperture of the diaphragm at different diame- 

 ters, and with different speeds, will develop with practice. The 

 following may serve as a general rule. A wide opening of the 

 diaphragm necessitates less time than a narrow opening, but the 

 sharpness of the negative is proportionately decreased; and a 

 narrow opening of the diaphragm necessitates more time, and the 

 sharpness of the negative is proportionately increased. Some 

 form of exposure meter is a great help in determining the correct 

 time of exposure. 



What Shall Be Our Policy Concerning Gardening in 

 the Elementary City Schools?* 



By C. D. Jarvis 



U. S. Bureau of Education 



To vitalize their school studies, or to facilitate the acquirement 

 of a knowledge of the essentials in education, children need some 

 kind of active experience in the affairs of life. Before the excessive 

 centralization of population such experience was provided by the 



*Paper presented at the Annual Meeting of the Am. Nat. Study Society, 



Columbus, Ohio. 



