PROCESSING BT SLIDES 
As soon as the slide is removed from the BT, ex- 
amine it to be sure that a suitable trace has been 
obtained. Sometimes the smoked surface of the slide 
washes away on contact with sea water. If this has 
occurred, lower the BT again with a fresh slide. If it 
reoccurs, try a fresh lot, and handle the defective 
slides in accordance with Bureau of Ships instructions. 
With a sharp instrument, write the following infor- 
mation on the slide, being careful not to obscure or 
touch the temperature-depth trace. 
(1) Slide number. Number slides consecutively be- 
tween ports. If a ship is at sea for a week and uses 
forty slides, they should be numbered from 1 to 40. 
(2) Time group. Use GreenwichMeanTime (0001 to 
2400), giving the minute at which the BT entered the 
water. 
(3) Date. Day, month, and year in numerals. Use 
Roman numerals for the month. 8 September 1950 
is written 8 IX 50. 
(4) BT number. The serial number of the BT is 
stamped near the nose of the instrument. This number 
is very important, as the laboratory that will process 
your slides has hundreds of grids, only one of which 
is a duplicate of yours. Without the proper grid, 
the information on your slide is worthless. 
Always enter the information in the order given here. 
Avoid the temptation to improve an apparently faint 
trace by enlarging or tracing over it at the time you 
enter the data. The processing laboratory can copy 
an actual trace, however faint, by the delicate photo- 
graphic processes it uses, but will invariably detect 
a retouched trace and reject it as spurious. 
To fix the slide, first rinse off the sea water by dipping 
the slide gently once in clean fresh water. Then, dip 
it gently in the fixing lacquer, allow the excess lac- 
quer to drain off, and place the slide in the slide box. 
To prevent the slides from sticking to the box, it is 
good practice to put a length of string or strip of 
blotting paper under them, and to move each slide 
after about an hour to free it 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 re- 
move them. 
CAUTION: The BT lacquer is inflammable; it should 
not be used in the presence of lighted 
cigarettes. Since the lacquer evaporates 
quickly and is sometimes hard to procure, 
the dipping jar should be closed as soon 
as possible after the slide has been 
dipped. A mixture of one part lacquer 
to two parts thinner usually gives the 
-proper thickness for BT slides. 
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