eral objectives of the same formula, made at the 

 same time there will be such great differences that 

 it can hardly be conceived on the first examination, 

 that they were to be similar. It is at this point 

 especially necessary to detect the errors, to deter- 

 mine their cause and apply the remedy, and to do this 

 properly often involves an inconceivable amount of 

 work, and in many cases the final results are reached 

 at a pecuniary loss. There are certain fixed tests for 

 each kind of objective, and to the best of my knowl- 

 edge all reputable opticians bring each objective up 

 to its standard before allowing it to pass their hands, 

 irrespective of the cost of doing_so. This must of 

 necessity be so, if only out of business consideration, 

 and not for a love of each production, for it is evi- 

 dent that a well-earned reputation would soon lose its 

 pre-eminence, and would acquire one for unreliable or 

 poor work, if on comparison, objectives of the same 

 kind would show a marked difference. There is some- 

 times a fortunate combination of circumstances which 

 makes a certain objective better than its fellows, but 

 this is a rare exception, and is positive evidence that 

 the exact requirements of the formula have been com- 

 plied with. As a rule, therefore, I believe that the 

 opticians' claim may be relied upon, and where the 

 results in the hands of the microscopist do not corres- 

 pond with them, .the cause may usually be looked for 

 in the lack of experience in manipulation or in con- 

 ditions, which differ from those under which the objec- 

 tive was completed. The belief, which I am aware is 



