128 BACTERIAL DISEASES OF PLANTS 



much labor and should be carefully protected (covered) until 

 engraved. 



Very much can be done by line work alone, but the finest 

 results, especially where details are important, can be achieved 

 only by making the drawing partly or wholly of large and small 

 dots (stipple) , placed close together and wide apart as the shading 

 or other features require. As such drawing is rather hard 

 on the eyes it should generally be done under a drawing glass 

 magnifying two or three times, and always should be drawn 

 larger than the desired reproduction so that small inequalities 

 in the hand work may disappear or blend into a general smooth 

 whole when it is reduced one-half or two-thirds. Great care 

 must be taken to lay the ink on properly so that the reduction 

 shall look uniform and attractive. Such drawings may be made 

 direct from the microscope or inked over a pencil sketch, or 

 may be laid over a pale silver print on salted paper, or a blue- 

 print, or a print on bromid paper, the remaining photographic 

 details of which are then bleached out in a bath (the importance 

 of the water-insoluble India ink is now apparent) after w^hich the 

 sheet is dried and retouched here and there, as necessary, to 

 fill in places which were overlooked. 



The bleaching baths are as follows : 



1. To bleach silver-salted paper: use a weak solution of 

 mercuric chlorid in alcohol (poison). 



2. To bleach blueprints: use a dilute solution of potassium 

 carbonate and potassium hydrate after which the print should 

 be flowed with dilute hydrochloric acid and washed. 



3. To bleach bromide prints: 



a. Use Thiocarbamide 120 grains. 



Nitric acid 2 fluid drams. 



Water 10 ounces. 



Wash and dry. Safe but slow. 



or h. Use a saturated solution of iodine in 



alcohol 2 drams. 



Saturated solution of potassium 



cyanide in water 3 drams. 



By adding water the action becomes slower. 

 Note. — This solution is very poisonous. 



