TANK WINDOW 



The oceanographic instruments being calibrated 

 are viewed through the tank window (fig. 3) which measures 

 9 by 19 inches and is made up of four panes of 1/4-inch glass, 

 opaque to infrared. The three dead-air spaces between the 

 panes make conductive heat transfer negligible, and the 

 optically ground glass gives undistorted visibility. Orig- 

 inally a wooden frame supported the therm.opane, but 

 alternate drying and soaking caused variation of its dimen- 

 sions and the seal developed a leak. A Micarta frame, 

 impervious to water, was substituted and no leaks have 

 developed in several years of use. 



TRAVELER, 



HORIZONTAL AND VERTICAL 



F" -^-^^"^^ READING TELESCOPE 



VIEWING WINDOW 



Figure 3. Viewing window and telescope unit. 



