_Willesden Paper._--When paper is passed through an ammoniacal solution of copper oxide, a superficial gelatinisation of the surface takes place, so that the paper when washed and dried is impregnated with copper oxide, which helps to preserve it, and it becomes waterproof. Such material is well known as Willesden paper. _Blue Print or Cyanotype Papers._--This name is usually given to the process by means of which blue prints of engineers' and architects' plans can be reproduced. It was discovered in 1842 by Sir John Herschel. It is a useful method of reproducing drawings, and incidentally is of great value to the amateur photographer because of the facility with which it can be applied for getting proofs from negatives quickly and easily without special baths and chemicals. The process is based upon the reduction of a ferric salt to the ferrous condition by light, and the formation of Prussian blue by the action of potassium ferricyanide. The _negative cyanotype_ gives white lines on a blue ground. Various formulæ are in common use. ----------------------------+---------+--------+--------+--------- -- |Herschel.| Clark. | Watt. |Rockwood. ----------------------------+---------+--------+--------+--------- | | | | Solution 1. | | | | Potassium ferricyanide | 16 | 27 | 48 | 10 Water | 100 | 100 | 100 | 100 Ammonia | -- | 2·3 | -- | -- Saturated solution of | | | | oxalic acid | -- | 20 | -- | -- ----------------------------+---------+--------+--------+--------- | | | | Solution 2. | | | | Ammonia-citrate of iron | 20 | 30 | 50 | 30 Water | 100 | 100 | 100 | 100 Boric acid | -- | -- | 0·5 | -- Dextrin | -- | -- | -- | 5 ----------------------------+---------+--------+--------+--------- Equal parts of the two prepared solutions are mixed when required and spread evenly over well-sized paper. The paper is hung up, dried, and preserved in a dark dry place. The _positive cyanotype_ gives blue lines on a white ground, being the reverse of the ordinary blue print. That is, no image is formed where the light acts, and the reaction is the formation of blue due to the union of a ferrous salt with ferrocyanide of potassium. Pizzighelli in 1881 gave the following formula:--