168 



METHODS IN PLANT HISTOLOGY 



orounef f/ass 



bound together along one edge with a piece of lantern-sUde binding 



tape. 



The relative positions of light, ground glass, negative, and paper 



are shown in Figure 32. 



A 200-watt light or, better still, four or five 60-watt lights, will be 

 sufficient for most work. There should be a reflector behind the Hght. 



The ground glass, about 2 inches 

 from the negative, is to diffuse the 

 Hght. 



Almost any camera which takes 

 ordinary plate holders will do, but 

 we should advise a strong 5X7 

 camera of the general Primo pat- 

 tern. The vertical position is bet- 

 ter, because the paper is easier to 

 manipulate. If you prepare your 

 own apparatus, it will save time 

 to look at enlarging cameras on 

 the market, or at least to study 

 the illustrations of them in cata- 

 logues. 



The time of exposure will vary 

 with the light, the negative, the 

 paper, and the amount of enlarge- 

 ment. With a 200-watt bulb, or a 

 group of 60-watt bulbs, a good 

 negative, an average paper, and 

 an enlargement of three diame- 

 ters, try 20 seconds. It will save 

 time and money if you use a full 

 sheet of paper and expose for 5 seconds; then cover a strip about an 

 inch wide with a piece of black paper or a metal squeegee plate 

 and expose the rest for 5 seconds more; then cover another inch and 

 expose for 5 seconds; and so on across the paper. When you develop, 

 you will see which exposure was best and, after a few trials, you will 

 be able to estimate the exposure without any trial or, at least, will be 

 able to estimate by trying a small piece of paper. 



There are various developers, but the one given below seems to be 

 as good as any. 



foble 



Fig. 32. — Relative positions of reflector, 

 light, ground glass, negative, lens, and table. 

 At top, above light, there should be holes for 

 ventilation. 



