OF A.RTS AND SCIENCES. 161 



The instrument is now pointed toward the zenith for about eight 

 minutes, on a day when there is a bright blue sky. On taking the 

 apparatus into the dark room, and viewing the impression by gas-light, 

 it will be found that the markings, which are cpxite clear at one end, 

 have entirely faded out by the time the middle division is reached. 

 The last division clearly marked is noted. Five strips cut from sensi- 

 tized glass plates, ten centimeters long and two and a half in width, 

 are now placed side by side under the scale, in the place of the chlo- 

 ride. By this means we can test, if we wish, five different kinds of 

 plates at once. The cover of the sensitometer containing the .05 cm. 

 hole is put on, and the plates exposed to sky light for a time varying 

 anywhere between twenty seconds and three minutes, depending ou 

 the sensitiveness of the plates. The instrument is then removed to 

 the dark room, and the plates developed by immersing them all at once 

 in a solution consisting of four parts potassium oxalate, and one part 

 ferrous sulphate. After ten minutes they are removed, fixed, and 

 dried. Their readings are then noted, and compared with those ob- 

 tained with the silver chloride. The chloride experiment is again per- 

 formed as soon as the plates have been removed, and the first result 

 confirmed. AVith some plates it is necessary to make two or three 

 trials before the right exposure can be found, but if the image disap- 

 pears anywhere between the second and eighth divisions, a satisfactory 

 result may be obtained. 



The plates were also tested using gas-light instead of daylight. In 

 this case an argand burner was employed, burning 5 cu. ft. of gas per 

 hour. A diaphragm 1 cm. in diameter was placed close to the glass 

 chimney, and tlie chloride was placed at 10 cm. distance, and exposed 

 to the light coming from the brightest part of the flame, for ten hours. 

 This produced an impression as far as the third division of the scale. 

 The plates were exposed in the sensitometer as usual, except that it 

 was found convenient in several cases to use a larger stop, measuring 

 .ol6 cm. in diameter. 



The following table gives the absolute sensitiveness of several of 

 the best-known kinds of American and foreign plates, when developed 

 with oxalate, in terms of pure silver chloride taken as a standard. 

 As the numbers would be very large, however, if the chloride were 

 taken as a unit, it was thought better to give them in even hundred 

 thousands. 



VOL. XX. (N. S. XII.) 11 



