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The. Best, the most Simple, and Unerring Tests for 

 Objectives. 



By William Webb. 



{Read December 27, 1872.) 



In submitting this paper for your consideration, I pray that so 

 much of the verbiage as prima facie may appear to be egotistical 

 or presumptive may be treated with kindness, and not allowed to 

 prejudice your minds until the whole paper has been read, discussed, 

 and calmly considered. A double apprenticeship to the study and 

 practice of the subject enables me to speak in terms so confident 

 and positive that I fear to give offence, even in the initiatory title, 

 viz. : — " The best, the most simple, and unerring tests for ob- 

 jectives." 



Apologizing, in this company, for saying what is required of a 

 .Test is of Definition, Flatness of Field, and Distortion. 



In speaking of definition in most instances I have adopted 

 square measure, but where practicable I have expressed my words 

 in lineal measure. 



To view distinctly the five thousand millionth of an inch is 

 good definition. To view the same space with equal distinctness 

 all over the field is flatness of field. To view an object, and to 

 find it presenting an abnormal state, is distortion. 



I now propose to treat the Definition and Flatness of field 

 together, and to submit that there is no test so certain as a series 

 of engravings on glass. For my purpose I engrave a series of 

 plates with letters measuring from one two hundred thousandths of 

 an inch to one two hundred millionths of an inch. Each engraving 

 is of the Lord's Prayer, varying only in size, commencing about the 

 thousandth of an inch, which is at the rate of over a quarter of a 

 million letters to the inch, and progressively decreasing the size, 

 the next of the series being at the rate of a million letters to the 

 inch, the next two millions, the next three, and the next four mil- 

 lion letters to the inch. Having reached this point, and finding 



JouRN. Q. M. C. No. 21. I 



