1875.] Suggestions for the Yunan Exp edition. 13 



baths of distilled water for 5 minutes each. (If distilled water is not easily 

 available, common water, if tolerably pure, may be substituted for the dis- 

 tilled water in the last three baths), and then immersed in a dish containing 

 the following preservative carefully filtered, or if this is inconvenient, the 

 mixture may be poured on and off the plates once or twice, taking care to 

 avoid bubbles. 



Decoction of Coffee, 2 pints (1 ounce of coffee.) 



Sugar, 3 drams. 



White of one egg. 



The plates are allowed to remain a few minutes in this and then removed 

 and set up to drain on clean blotting paper and allowed to dry in a box or 

 other place free from dust. Plates so prepared are said to keep for some 

 months in Europe, but'I have no experience how long they would keep in 

 an Indian climate. 



III. Exposure. 

 For the exposure no certain rule can be given, but I find the plates 

 require about twice or three times the exposure of wet plates, and about 1 

 minute would probably be the exposure for small plates, such as would be 

 carried on an expedition, travelling as lightly as possible. This, however, 

 must be tested beforehand. 



IV. Development. 

 The plates are first of all flushed with a mixture of equal parts of 

 spirits of wine and water, and then well washed with clean water. 



Supposing the plate to be \ size, or 4|" X 3i", take 2 drams of a solu- 

 tion of — ■ 



I. — Pyrogallic acid, 12 grains. 



Water, 1 ounce. 



and pour it over the plate and back into the developing cup, then add two 

 or three drops of a mixture of 



II. — Strongest Liquor Ammonia, 1 part. 



Water, 4 parts. 



and one drop of 



III. — Bromide of Potassium, 4 grains. 



Water, 1 ounce. 



and again apply to the plate. If a proper exposure has been given, the 

 details will at once become visible and gradually gain strength, till all 

 possible detail is out. Should, however, the plate have been under exposed, 

 and the details be slow in appearing, a drop or two more of No. II. 

 must be added, but if the plate has been much over-exposed and the 

 details flash out, it may be advisable to at once add a drop or two 

 of No. III. 



