314 0)1 a New Critical Standard Measure [^f^Snla^Krim' 



with the naked eye 112 Hnes drawn to the inch, placed at ten inches' 

 distance. Counting them is another thing altogether. 



Since writing the ahove remarks, I may add that a careful ex- 

 amination of Dr. Colonel Woodward's wonderful photographs of 

 Nohert's lines, including the XlXth hand, shows spurious lines at the 

 left hand of the striated bands much more distinctly delineated than 

 the hand itself. Indeed, the XlXth appears flecked with a peculiar 

 waviness, mottled as it were with the tremulous motion of a heated 

 stratum of air as seen through a land telescope on a hot summer's 

 day. The lines are there indeed, hut more or less continuous. I 

 am of opinion that were Dr. Woodward in possession of the |th or ^^th 

 recently constructed by Messrs. Powell and Lealand, either as dry 

 or water lenses, the lines of the XlXth hand would now be pictured 

 as sharp and clear as the spurious lines at the edges, which wiU be 

 probably reduced by a better definition. 



Dr. Woodward distinctly states that Mr. Stodder and Mr. Green- 

 leaf were not able to count the lines. So Iowa critical angle as 100 

 seconds, indeed, could hardly enable the acutest sight to perform 

 Buch an optical feat. Our distinguished President informed me he 

 could see telegraph wires J inch in diameter at 800 yards' distance, 

 or a subtense of one in 115,200 : or as one in 206,270 nearly equals 

 one second, the telegraph wire would thus be slightly under two 

 seconds. We may conclude that very keen eyes may see a line 

 only two seconds in diameter. Nohert's XlXth band, with a power 

 of 550, gives a hundred seconds between the centres of each con- 

 tiguous line, yet it required a power of 1000 diameters and a critical 

 angle of 183" to enable Dr. Woodward to so discern the line as to 

 be able to count them with certainty with our best rijth immersion of 

 that date. Without entering into any description of the optical 

 causes which render these hnes so indistinct, even at 200 seconds of 

 separation in the microscopic field, an interesting account might be 

 given of the sensibility of the eye as affected by " personal errors 

 of observation," and the ordinary discriminating vision of average 

 observers. 



But any one who can count the lines with 250 on the Angula- 

 tum will perform almost the identical feat claimed by Mr. Stodder. 

 For this gives nearly a critical angle of 100", reckoning the hnes of 

 Angulatum 52,000 per inch.* 



There is nothing in the nature of vision to divorce microscopical 

 from telescopic fields of view. In both cases the final image is 

 presented to the eye for vision by the eye lens at its focal points in 

 the plane of the stop. In both the actual image is seen not close, but 

 a distance of ten inches, more or less, from the eye. The experience 

 of astronomers therefore gives a rich fund of data for estimating 



^^ 20627x251-85 ^^^._ . 



'= ^12000 =100-1 nearly. 



