IOWA ACADEMY OF SCIENCES. 43 



nut G is released, serves to enable the operator to move 

 the object holder along the table, by means of a rod E 

 lying under the slot. This clamp, like the one at C, Fig. 

 1, consists of a piece of wood about six or eight inches 

 long. It has a cross section somewhat resembling an 

 inverted capital T, as shown just above E, Fig. 3. The 

 tongue projecting up into the slot keeps the clamp in 

 alignment. The screw or bolt G passes through the body 

 of the clamp and up through a hole in the base of the 

 object holder. And when the latter is to be clamped fast 

 the nut is simply tightened down. During the process of 

 adjustment the nut is kept loosened so that the clamp may 

 slide freely in and under the slot. The rod E by which the 

 operator moves the clamp, and with it the object holder, 

 is oblong in cross section. It lies under the slot in the 

 table top and reaches the whole length of the table. 

 Screwed to the under side of the body of the clamp is a 

 loop of plate brass; and in Fig. 3 the rod E lies, thrust 

 loosely through this loop. If it be now twisted some 40 

 degrees it will bind in the brass loop and so engage the 

 clamp. The latter, with the object holder, can then be 

 drawn back or thrust forward at the will of the operator. 

 The rod may be disengaged from the clamp by a turn of 

 the hand, and may be left, pushed in under the table, out 

 of the way, when the adjustments are finished. 



By the arrangements here described all adjustments of 

 the camera and object may be made without removing 

 one's head from under the focusing cloth. Any one who 

 has used a camera for the purposes mentioned here knows 

 that all the adjustments of this apparatus are desirable. 

 First, the object to be photographea is placed approxi- 

 mately in the right position on the object holder. The 

 exact vertical and horizontal adjustments are next made; 

 and finall}^ any fault in the orientation of the image on 

 the ground glass is corrected by revolving the camera 

 about the axis of the lens. When the image is of the 

 right magnitude and in proper position the movable parts 

 are fastened in their positions by the clamping screws or 

 nuts. 



