Figure 2-13. Labeling the BT .slide. 



(3) Date. — The third Une is the day, month, 

 and year in numerals. Use Roman numerals for 

 the month. Thus 13 August 1955 is written 

 13-VIII-55. 



(4) BT number. — Each BT has its own 

 number. The BT number is stamped near the 

 nose of the instrument and also is recorded on 

 the grids for the shdes. This number is very 

 important since the laboratory that will process 

 the slides has hundreds of grids, only one of 

 which is a duplicate of yours. Without the 

 proper grid, the information on the slide is 

 worthless. Prefix the number with BT, thus 

 BT 1591. If there is a letter immediately follow- 

 ing the number always include it, thus BT 

 780A. Always enter the above information in 

 the order given. 



Avoid the temptation to improve an appar- 

 ently faint trace by enlarging or tracing over 

 it at the time you enter the data. The process- 

 ing laboratory can copy an actual trace, however 

 faint, by the delicate photographic processes 

 it uses, but will invariably detect a retouched 

 trace and reject it as spurious. 



2-50 Lacquer and Store the BT Slide.— After 

 the BT slide has been labeled, hold the slide 

 with the dipping tongs provided and rinse it 

 by dipping in a jar of fresh water. Let the slide 

 dry and then gently dip it in thin lacquer. Let 

 the excess lacquer drain against the lip of the 

 jar or by standing on end on absorbent paper 

 for several seconds. The slide may be dried by 

 holding it near a light bulo or some other source 

 of dry heat. On survey ships where BT observa- 

 tions are taken very frequently a drying box 

 containing a light bulb is used. The slide is 

 left in the dipping tongs and the tongs are 

 suspended from a hook near the bulb until 



H. O. 607 



thoroughly dry. The slide is then placed in 

 the storage box. 



If the slide is left to dry in damp moving air 

 the lacquer often fogs, or turns white, and the 

 slide is difficult to read. A fogged slide may be 

 cleared, however, by redipping it in lacquer 

 after it has dried. 



If a source of drying such as described above 

 is not available, it is best to place the slide in 

 the storage box, immediately after draining 

 the lacquer, and close the lid. To prevent the 

 slides from sticking to the box it is good practice 

 to put a length of string, wire, broom straw, or 

 a strip of waxed paper lengthwise under the 

 slides in the bottom of the box. After about 

 an hour, open the box and move each slide 

 before the lacquer is fully dry and hard. If 

 the slides are permitted to stick solidly to the 

 box, the chances are that many of them will 

 be broken when the time comes to remove them. 



CAUTION 



BT lacquer is inflammable; it should 

 not be used in the presence of lighted 

 cigarettes or other open flames. The 

 jar of lacquer should be closed im- 

 mediately after using since it evapo- 

 rates rapidly and is highly flammable. 



After drying, the lacquer on the slide should 

 be perfectly transparent, hard, even, and just 

 thick enough to protect the smoked surface 

 from being scratched during ordinary handling. 

 If evaporation has occurred, the coating on 

 the slide will be thick and uneven, and thinner 

 must be added to the lacquer. The lacquer 

 should be kept at a consistency only a little 

 thicker than water. 



Acrylic spray (Krylon), applied from a spray- 

 bomb-type container, has been used successfully 

 to coat slides. 



2-51 Viewing the Slide. — Place the long 

 edge of the slide having the smaller bevel 

 against the spring at the bottom of the grid, 

 with the smoked surface toward the grid and 

 the edge with the beveled corners just clearing 

 the eccentric stop screw of the viewer. Gently 

 push the slide against the spring down into 

 place so the smoked surface lies flat against the 

 grid. Then push the slide snugly against the 

 stop. 



To remove the slide from the grid depress the 

 spring with the fingernail to loosen the slide 

 from the grid mount. 



This method prevents excessive scratching 

 or rubbing of the smoked surface against the 

 grid. When the holder and the slide are 



23 



