NOBERTS TEST PLATES. 



a power of 450 will render the lines of the tenth band equally 

 visible ; indeed, it is not necessary at all to have recourse to the 

 microscope to ascertain the effect which a given magnifying power 

 ought to produce upon a band of a given degree of closeness, since 

 it is evident that the effect must be merely to make the lines com- 

 posing the bands more widely separated than they are in the exact 

 proportion of the magnifying power. Thus, if the lines composing 

 a band, separated by intervals of the 10000th part of an inch, be 

 viewed with the magnifying power of 100, they will appear as 

 those of a band separated by intervals of the 100th of an inch ; and 

 if it be viewed with a magnifying power of 1000, it will appear as 

 if the lines were separated by the 10th of an inch, and so on. 



Now, let us apply this obvious principle to the case given in the 

 report of the Juries ; a magnifying power of 100 directed upon the 

 first band, would make the lines appear as if they were separated 

 by intervals of the 112th part of an inch; those of the second 

 band would appear separated by intervals of the 131st part of an 

 inch, and those of the third by the 153rd part of an inch. Now, 

 all these would, as admitted in the report, be distinctly seen as 

 separate lines, by eyes of average power. But let us see what 

 effect a magnifying power of 2000 would produce upon the closest 

 of the bands. 



Since it would render the apparent intervals between line and 

 line 2000 times greater than they are, those between the lines of 

 the tenth band, would be the 25th; those of the ninth, the 19th; 

 and those of the eighth, the 17th part of an inch. 



Although it must be quite evident that such intervals are much 

 greater than is necessary to enable any eye whatever that can see 

 at all, to perceive the -lines distinctly separated, the reader will 

 be enabled better to appreciate the point by referring to the 

 numbers which we have placed on the right of fig. 24, which 

 express severally the number of lines to an inch in each of the 

 bands composing that figure ; thus, the lines of the bands B and c 

 are separated by intervals of the 48th part of an inch; and it 

 follows, therefore, that a magnifying power directed upon the band 

 x of the test-plate, mentioned in the report of the Juries, would, 

 if viewed by a power of 2000, show the lines separated by intervals 

 twice as great, or equal to those of every other line in the bands 

 B and c, fig. 24. 



For these reasons, it appears to me that a mistake has been 

 committed in the report of the Juries in this point, and I have 

 thought it the more desirable to call attention to it, inasmuch 

 as the .statement has been reproduced in several recent works 

 upon the microscope. 



It is easy to show what would be the degree of closeness of the 



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