36 Cincinnati Society of Natural History. 



will appear in the film. The image should appear in a couple of 

 seconds, and the development should be complete in from ten to 

 twenty seconds. The plate is then rinsed under the tap, and fixed 

 in a strong solution of hypo-sulphite of soda, after which it is 

 washed for twenty minutes, and then hung on a rack or dried by 

 heat. A better developer is Lea's Sugar Developer, made as 

 follows: In 32 oz. of hot water dissolve 7 oz. of protosul])hate of 

 iron, and add 6 oz. white sugar and 2\ oz. acetic acid, which 

 makes the stock. For use take : Stock 7^ oz., acet. acid No. 8. 

 4 oz., water 18 oz., filter. Add more acetic acid if there is any 

 sign of fogging. 



The collodion film is very delicate, and must at no time be 

 touched, or it will be scratched and spoiled. After fixing the 

 plates may be toned in various ways. I prefer a weak solution of 

 chlor. gold, about i gr. to 30 oz. water. This is flowed over the 

 plate several times, and requires only a minute or two to act. 

 Bichloride of mercury is often used, and gives a rich purple tone, 

 but I have found that plates thus toned fade considerably in the 

 course of a year or so. 



This may seem like a difficult process to those who have been 

 accustomed to the gelatine dry plates, but after the bath has been 

 made and a (juantity of plates have been cleaned and abluminized 

 the process is very rapid. I would advise those who try it to pro- 

 vide half a dozen finger stalls of thin rubber, as by using them tlie 

 silver stains, otherwise inevitable, will be avoided. Excellent dry 

 plates for lantern slides are made by washing and drying collodion 

 bath plates after putting them in a weak solution of acetic acid 

 and flowing over them a strong infusion of coffee. They are de- 

 veloped with pyrogallic acid and nitrate of silver, and give results 

 of the highest equality. They are generally used for printing by 

 contact, as in the camera they are very slow. 



I have used but two brands of commercial dry plates success 

 fully for lantern slides— the Anthony Transparency plates and Car- 

 bett's Gelatino-albumen. With careful handling these plates give 

 about equally good results, and both almost equal to the best to be 

 obtained by the wet-plate process. Each has, however, its own 

 peculiar advantages and faults. I have thought that the Anthony 

 plates are a little more easily controlled in case of over-exposure, 

 and the Anthony developer is simpler, as it may be made very 

 quickly from saturated solutions of iron and oxalate. The i)rinci- 



