166 METHODS IN PLANT HISTOLOGY 



The ammonium carbonate evaporates so rapidly that it must be 

 kept in closely stoppered bottles, preferably in emery stoppered bot- 

 tles. 



The different tones are obtained by mixing A and B in different 

 proportions. 



Cold sepia, 8 parts A to 1 part B. Develop, 5-8 minutes. 

 Warm sepia, 10 parts A to 3 parts B. Develop, 12-15 minutes. 

 Purple-red, 8 parts A to 5 parts B. Develop, 20-30 minutes. 



Do not try to shorten the time of development by giving a longer 

 exposure. 



Rinse thoroughly and fix in an acid fixing bath. The following is 

 recommended for all plates: 



Hypo 150 g. 



Sodium bisulphite 15 g. 



Water 1,000 c.c. 



MAPS AND GRAPHS 



One often needs a lantern slide of a map. An easy but very unsatis- 

 factory method is to copy from an atlas or, not quite so bad, from a 

 base map. The atlas will have so many places named that what you 

 want to show will be obscured. The best way is to trace a rather large 

 map of the desired region, rub the underside of the tracing paper with 

 a soft pencil and then, by following the lines with a hard pencil, trans- 

 fer the outline to pure white cardboard. Ink it with dead black India 

 ink making the lines very bold. If the map is 38 cm. wide, hnes 1 mm. 

 wide are not too coarse for the outlines of coasts and islands. When 

 they appear on the lantern slide, they will not be more than 0.2 mm. 

 wide. Lettering should be correspondingly bold. Letters less than 

 1 cm. high on a map 38 cm. wide will not be very conspicuous when 

 they appear on the screen. 



In copying a map 38 cm. wide, using a contrast lantern-slide plate 

 (red label plate, if Cramer's is used), with F16 in fair diffuse light, try 

 20 seconds. If such work is always done in the same place, you will 

 soon know whether to use 15, 20, or 30 seconds. If artificial light is 

 used, there should never be a mistake after you have once determined 

 the proper exposure. 



Develop in a contrast or process developer. Use hydrochinon : there 

 should not be any metol in the formula. 



Maps are more satisfactory when colored, if there are both land and 



