478 MANIPULATION. 



this glass is placed under a magnifying power of about 100 

 diameters, the bands containing the fewest number of lines 

 in them will present the appearance shown by fig. 262, in 

 which are exhibited the lines as seen in four of the coarsest, 

 the other six with so low a power not being visible, and even 

 those in the fourth band requiring some care in the illumina- 

 tion to define them satis- 

 factorily. In order to use 

 this test, the bands are 

 viewed by glasses of dif- 

 ferent focal lengths, in 

 the same manner as any 

 'S" ■ other lined objects, and 



the number of the bands with their lines clearly defined, wUl 

 form a good criterion of the merits of any magnifying power 

 from 100 to 2,000 diameters. Thus, for instance, if a quarter 

 of an inch object-glass be employed with the best iUumination, 

 nine of the bands may be seen, and the lines in seven of them 

 clearly defined, but stiU no trace of the tenth band visible ; if, 

 however, a twelfth is used, the lines in the tenth may be 

 shown ; and these, although the jj^ of an inch apart, are as 

 perfectly etched as those in the first band, which are seventy 

 times as coarse as those in the tenth. Of all the tests yet 

 found for object-glasses of high power, this would appear to 

 be the most valuable, and one which comes the nearest to the 

 utmost limit at which the position of a line xjan be accurately 

 ascertained. M. Nobert's paper is published in Poggen- 

 dorfs Annalen for 1846 ; but as it would be foreign to the 

 object of this work to enter so scientifically into the explanation 

 of the reasons for adopting this valuable form of test glass as 

 M. Noberfc has done, the reader is referred to the paper itself, 

 which will well repay an attentive perusal, as the information 

 it contains is of the highest practical importance. Accompa- 

 nying the test kindly lent to the author by Dr. Bennett, was 

 another glass, on which were etched in a similar manner a 

 series of lines, the 4^^ of a millimetre apart ; these were, like- 

 wise, beautifully ruled, and the surface of the glass presented 

 a rich play of iridescent colours. 



