DEFECTS IN NEGATIVES. 141 



Many of these changes and agencies have only 

 lately been discovered, and a very little experience 

 will convince us that others exist, which owing to a 

 want of continuity between cause and effect, are in- 

 capable of demonstration in the present state of our 

 knowledge. Some are certainly dimly present to 

 our minds, and although not fully recognised, are 

 allowed for by an unconscious cerebration, if we may 

 be allowed the term. At the same time the direction 

 in which further experiment is required, will be in- 

 dicated further on. 



The best method of arriving at a knowledge of 

 how to produce good negatives, is by studying the 

 defects of bad ones, endeavouring at the same 

 time to discover the causes producing these defects 

 and the remedies for them. 



Inaccuracy in the preparation of solutions and 

 want of manipulative skill, are the principal diffi- 

 culties besetting a beginner. It will only be neces- 

 sary to mention where and how these inaccuracies 

 are most liable to occur, to enable the manipulator 

 to guard against them. 



When distilled water is mentioned never use any 

 other ; pure spring water, not rain, being employed 

 for its production. Neutral solutions must be tested, 

 and what are sold as neutral salts can never be taken 

 upon trust, but must be made so. Obtain pure 

 chemicals ; see that all solutions are of the proper 

 strength; keep all bottles requiring it, tightly 

 corked ; and filter solutions when directed. Have 

 the dark room well ventilated, and on no account 



