WILLIAM WEBB ON THE BEST TESTS FOR OBJECTIVES. 115 



interesting part of the subject which deals with the tests un- 

 blackened. For this purpose I must go back to the square of the 

 line forming the letter as the 5,349,720,000th of an inch that, re- 

 duced to its square root, gives 73,000 + of an inch linear as the 

 breadth of the line. 



I mount the same series of slides in the way that Monsieur 

 Nobert mounts his justly celebrated tests — without black — and 

 thus open u^ a wonderful means of study of the whole subject, 

 helping to afford the power of determining at what breadth un- 

 blackened lines become invisible, even when aided by the micros- 

 copes of the present day. In this instance the 73,000th is an 

 absolute line, unbroken by a next line. 



When viewing the black lines ordinary direct illumination is 

 sufficient, but when examining the unblackened lines it becomes 

 necessary to adopt in its turn every available means of illumina- 

 tion, because the cut, being wedge-shaped, each side of the cut, 

 from every part to its very apex, both refracts and reflects again 

 and again the light from the other. Again, the original upper 

 and lower surfaces of the glass refract and reflect the light back- 

 wards and forwards ; again, the top light flows into the cut, help- 

 ing to produce the climax which blazes away the cut as the light 

 of the sun overpowers or destroys the light of a candle. 



By testing by blackened and by plain unblackened letters, it will 

 be found at what point the power of certain objectives ceases to be 

 effective with transparent objects. I can define the smallest Lord's 

 Prayer when blackened, that is, I can define a line of the 73,000th 

 of an inch, but have never been able to define the same test un- 

 blackened. More than that, although I know the exact spot that 

 it occupies, and mark the spot with an Indian ink ring before it 

 leaves the machine in which it is engraved, I have never (perhaps 

 because of irritable temperament) been able to discover not merely 

 the line, but the aggregation of lines forming the 227 letters of 

 the very small tests, although they become perfectly distinct when 

 black. 



If I shall be honoured by a full and exhaustive discussion of this 

 paper I may be delighted to submit to the Club another paper upon 

 the clever productions of M. Nobert, in the hope that I may be 

 able in some way to assist others who may not so fully have studied 

 his wonderful works, in arriving at a just appreciation of M. 

 Nobert's extraordinary, patient, and persevering skill. 



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