7i J. Waterhouso — The Application of Thotography [No. 2, 



one or two dishes or trays. The sensitive paper is easily prepared and can 

 be kejit indefinitely in the dark until required for use. There is no messing 

 with chemicals after the preparation of the paper, pure water only being 

 required to develope and fix the prints. The exposure to the light is very 

 short, two or three minutes in the sun being am^^le to make a clear legible 

 copy from a line negative or from a drawing on tracing cloth. The chemi- 

 cals employed are both very inexpensive. 



The objections to the process are two : first the difficulty of obtaining 

 clear whites ; this, however, is of no consequence so long as the details are 

 clearly legible ; and secondly, the colour of the prints — white on a dark 

 blue ground. Although this does not interfere with the practical use of 

 the process for special work, it completely prevents it from being employed 

 as a means of multiplying copies of maps or plans on a large scale. An- 

 other defect is, the want of sharpness arising from the necessity for placing 

 the reverse side of the original in contact with the sensitive paper in 

 order to get an unreversed print. These objections may be partly obviated 

 by printing from a negative on paper or glass, in which case the lines will 

 be dark blue on a light blue or w^hite ground, but then cameras and other 

 expensive photographic apparatus will be required to produce the negative. 



M. H. Pellet has recently recommended a process of this kind whereby 

 prints are obtained in dark lines on a clear ground. Paper is sensitised in 

 a mixture of — 



Oxalic acid, ... ... ... 5 parts 



Perchloride of Iron, ... ... ... 10 „ 



Water, ... ... ... 100 „ 



dried and exposed as usual under a drawing. The print is developed ii> 

 a bath of yellow prussiate of potash at 15 or 18 per cent, well washed and 

 fixed with dilute muriatic acid, then finally washed and dried. • 



The blue prints thus produced can also be used as the basis of 

 drawings for photozincography. 



Another process, which, though not quite so simple as the above, has 

 the advantage of giving a print in black on a white ground, forms one of 

 the numerous important photographic methods for which we are indebted 

 to the illustrious Poitevin. 



Paper is coated in the dark with a solution of perchloride of iron and 

 tartaric acid in water ; when dry, it is exposed under a tracing on cloth or 

 paper, or a reversed positive on glass, and as soon as the parts exposed to the 

 light have become thoroughly bleached the print is removed and developed 

 in a bath of gallic acid. The parts protected from the light turn to an inky 

 black, while the exposed and bleached parts remain white or only take a 

 slight tint. The print is then thoroughly washed and dried. The whole 



