which remove the effects of scale, tilt, tip, or swing. Large dif- 

 ferences in scale — up to 25 percent, for example — can be removed 

 by raising the transparent mirror on its pedestal mount and at the 

 same time raising the easel as in figure LA. Because this caused 

 the right-hand mirror arm on the stereoscope to interfere with 

 the line of sight, a new arm was made that would bypass the 

 mirror viewing area (fig. IE). However, when the transparent 

 mirror is raised to its highest point, images cannot be in focus 

 at the same time. Under these conditions the transfer must be 

 made without the aid of stereo, and the lights under the right-hand 

 member of the stereo-pair must be turned off. 



7. Blinking the light behind the easel aids greatly in getting 

 images to coincide exactly. 



8. Fine adjustments of the plot corner locations can be made 

 individually by using the X and Y parallax controls on the stereo- 

 scope. 



9. Corners of square or rectangular plots can be marked 

 permanently on the left-hand transparency with the sharp point 

 of a scribing tool. 



An interpreter using the Old Delft device can transfer and 

 interpret up to 50 plots per day. The principal advantages of this 

 deyice over the Ryker are (1) transfers are made in three dimen- 

 sions (when photo-scales are nearly the same), and (2) the 

 transparent mirror can be swung away from the optical path. Thus, 

 the plots located on the new photography can be interpreted without 

 making a new setup. 



THE RYKER DEVICE 



The primary difference between the Ryker (fig. 3A) and the Old 

 Delft device involves the stereoscope and its modifications. The 

 Ryker mirror stereoscope, Model M-ll, is mounted on an extended 

 bracket, and a "Giant" erecting prism substituted for the right-hand 

 mirror system (fig. 3B). The 16-tube fluorescent illuminator and 

 the upright easel with rear illuminator and foot switch are the 

 same for both devices. 



