PRACTICAL PHOTO-MICROGKAPHY. 147 



must be very good, flat, polished, free from scratches, bubbles 

 and other visible flaws. The glass must be cleaned, not only 

 in the usual acceptation of the word, but made chemically 

 clean by soaking for some hours in dilute acid (say nitric acid 

 1 part, water 10 parts). Thereafter it must be washed, prefer- 

 ably under a tap first, and then with a solution of sodic carbon- 

 ate. Next it should be well rubbed with a clean cloth dipped 

 in a mixture of alcohol and ammonia ; lastly, it should be pol- 

 ished with a perfectly clean and soft chamois leather. For 

 lantern slides we do not recommend any substratum of al- 

 bumen. , 



The collodion may be bromo-iodised, as generally sold and 

 used, but the iodide should be in strong proportion, the brom- 

 ide being here of less consequence. The collodion should be 

 rather limpid than thick, and should be moderately ripe, 

 neither quite fresh nor very high-colored with age. 



The " silver bath " may be the usual 35-grain one, and must 

 be acid, preferably nitric acid being used. Of course this bath 

 must have the usual trace of iodine, produced by immersion of 

 a coated plate for some hours, or by the direct addition of an 

 iodide. Either the flat or the dipping bath may be used, per- 

 haps the latter is here preferable, as dust must be most zeal- 

 ously avoided. Operations of coating and sensitising are 

 exactly as given in " Processes of Pure Photography," Chap- 

 ter YI. 



The developer may be the usual acid ferrous sulphate, or 

 the ammonio-sulphate, or the pyrogallol. The latter devel- 

 oper requires the longest exposure, but gives magnificent 

 results, especially if used with a collodion containing iodides 

 only. 



PYROGALLOL DEVELOPER. 



Pyro 2 grains 



Glacial acetic acid 30 minims 



Water 1 ounce 



Alcohol q. s. 



This solution should either be used fresh, or the pyro dis- 

 solved in alcohol in strong stock solution. The quantity of 

 alcohol, which should be of good quality, depends on the age 



