144 BACTERIA IN RELATION TO PLANT DISEASES. 



even when held direct!}' toward the sun. A cross-bar is screwed across the base 

 of the uprights of the window frame, 35 inches from the floor and a few inches 

 above the window silL To this bar a swing shelf is hinged and drops down out of 

 the way when not in use. This shelf is about 24 inches wide by 30 inches long. 

 When in use it is supported in a horizontal position by a removable leg. On top 

 of this shelf is placed a cracker-box or some similar box, to the sides of which, at 

 the bottom, beveled cleats are nailed, which slide through corresponding cleats 

 screwed to the top of the shelf This enables one to push the box toward the win- 

 dow or draw it back on a regular track. On the top of this box, at the back end, 

 or farthest point from the north window, the camera is placed facing this window 

 and is screwed fast to the top of the box the same as to a tripod. Sidewise move- 

 ment is provided by extending the screw-hole in the top of the box into a slot 6 or 

 8 inches long. In sliding the camera sidewise it is of course necessary to keep the 

 ground glass parallel to the negative in the window, and this is done by drawing 

 parallel lines on top of the box about 'a inch apart and exactly at right angles to 

 the negative-carrier. In mo\-ing the camera sidewise all that then remains is to 

 see that one side of the camera at front and back matches one of these ruled lines. 

 This gives ample sidewise movement, and some up-and-down movement is usually 

 provided in the camera itself The rest is obtained by moving the iregative. The 

 upper half of the north window is covered by the curtains. The lower part is filled 

 with a removable wooden framework, the negative-carrier, so arranged that the 

 negative itself may be moved up and down or sidewise, or twisted around at 

 will. The framework of the negative-carrier is made of inch stufi^. When in use 

 it is placed upright about 3 inches in front of the window pane and just behind 

 the cross-bar which keeps it in place. In the middle of this frame is a circular 

 wooden disc (which must turn freel}), held in place on the back by a ledge and in 

 front by four buttons, and open in the center. The breadth of this disc is 24 inches, 

 and it sliould be made of well-seasoned lumber of a sort not inclined to warp. On 

 this disc at either side two broad vertical cleats are fastened. These are grooved 

 on the inner edges next the framework, and under them, close to the circular piece, 

 two wide 3 8-inch pieces slide up and down freel)', carrying the negative between 

 them. The latter fits into marginal grooves and is held in place by buttons. The 

 marginal grooves extend the whole length of the ag-inch pieces and consequently 

 allow the negative to be moved sidewise to any extent desired, while the rrp-and- 

 down movement is obtained by sliding the two 3^-inch pieces and the negative 

 between them as a unit. Over the first grooved cleats, at right angles, /. <f., horizon- 

 talh', two similar cleats are screwed. These also have wide ^s-inch pieces moving 

 under their grooved edges. These sliding pieces cover the sides of the negative 

 and shut out the side-light in whatever jiosition the negative may be placed. Behind 

 the negative against the window is pasted (b)' its corners) a good-sized piece of 

 white tissue-paper, which serves to distribute the light evenly and to cut out images 

 of trees, buildings, etc. Wlien in use the double curtains of the west window are 

 drawn down and the door is shut. The north light which enters the room then 

 comes through the negati\'e placed in front of the camera. The focus is obtained 



