271 



PROGRESS OF MICROSCOPICAL SCIENCE. 



in question "will be found in tlic 'Proceedings of the Royal Society,' 

 No. 163, and it is by Mr. J. A. Bro^vn, F.R.S. 



The following table contains the results of the observations of 

 Nobert's test-lines. 



Measures of Nobert's Test-Lines. 



Band. 



Number! 



of 

 Lines. 



Widih of 



Space. 



Baud. 



Number to the Inch. 



Lines. Spaces. Both. 



I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



IX. 



X. 



XI. 



XII. 



XIII. 



XIV. 



XV. 



XVI. 



XVII. 



XVIII. 

 XIX. 



58-67 



23-00 



14-85 



15-57 



15-20 



13-90 



9-14 



10-71 



9-93 



7-44 



6-90 



7-28 



6-00 



6-00 



5-56 



553 

 560 



555 

 505 

 480 

 495 

 505 

 475 

 478 

 460 

 500 

 503 

 500 

 510 

 495 

 522 



540 



35,000 



28,500 



35,910 



52,270 



71,430 



86,580 



79,110 



114,810 



1.33,870 



123,300 



123,300 



146,840 



151,060 



166,670 



179,860 



17,040 



43,450 



67,340 



64,230 



65,790 



71,940 



109,410 



93,370 



100,700 



134.410 



144,930 



137,360 



166,670 



166,670 



179,860 



11,460 

 17,210 

 23,420 

 28,820 

 34,250 

 39,290 

 4.5,910 

 51,390 

 57,470 

 64,310 

 66,670 

 70,970 

 79,240 

 83,330 

 89,930 

 77,280 



77,880 



79,240 

 75,760? 



00 



68 



U 



10 



•87 



23 



65 



■49 



•92 



25 



25 



8-08 



8-89 



9-81 



10-50 



Notes. — These measures are frequeutly mere approximations ; and in several 

 bauds the graving-point has made a wonderful approach to an equality of width 

 of lines and spaces ; indeed, these lines are marvels of mechanical skill. If, in 

 the case of each band, the first and last lines had been drawn longer than the 

 rest, it would have been possible to measure the width of a line with considerable 

 accui-acy, since, as has been shown, the visibility of a single line is nearly twenty 

 times that for the series. 



The w idths of the lines and spaces are those taken from the photographs, the 

 unit being ywot, inch. The photographs are magnified to 1000 times. In bands 

 XVII. and XVIII. second measures are given from photogi-aphs magnifying to 

 1600 times (but reduced to the same unit). The number of Hues in ( ) are the 

 numbers counted for which the total width was measured. The number for the 

 XlXth is deduced from the measure of a few where the lines were most distinct. 

 The numbers of lines and spaces to an inch are the numbers which could be put 

 in an inch laid side by side (without interval). Under "Both" is given the 

 number of lines to the inch (with interspaces), as in the bands. The " Eatio " 

 is that of the number for the widest space (17,000 to the inch) to the number for 

 the widest line or space in the following bands. 



It will be seen that the least width of the lines which can be 

 counted and measured on the photographs is about l eo^ao of an inch 

 (Xlllth band). We have seen (5th observation) that dark parallel 

 lines on glass can be seen with transmitted light when their width 

 subtends an angle of 20" to 26" ; so that lines stopping the light 

 moderately (7th observation) of t^cjV^o of an inch wide should be 



