Notes. 393 



spectrum beyond the bands above mentioned. This is still the 

 case when this part of the spectrum is rendered much more 

 intense by the use of iron, instead of carbon poles. 



The ultra-violet carbon spectrum falls off very much in in- 

 tensity beyond the second band, there being only one other band 

 (at about its middle) at all comparing in brightness with the first 

 two. These two bands, however, reach the limit transmitted by 

 most glasses. 



On Grayson's 120,000 Band-Plate. 

 By Edward M. Nelson. 



Through the kindness of Mr. Beck I have been enabled to examine 

 a Grayson Test-Plate, which contains a band ruled at the rate of 

 120,000 lines to the inch (= -21 /a). This band was strongly 

 resolved by an apochromatic oil-immersion £ 1*43 1ST. A. (for the 

 long tube) and a 5 eye-piece. With 12 and 24 eye-pieces the 

 resolution was quite as strong as the transverse stripe of the 

 Amphipleiira pellucida appear in good photomicrographs. 



This band was also resolved by a semi-apochromatic ^ 1*3 N.A. 

 (for the long tube) with a 5 eye-piece. A friend even saw the lines 

 with a 4 eye-piece, though I could not do so myself. 



It was also resolved by an old achromatic water-immersion 

 T T 2 1 ' 2 1ST. A. by Powell and Lealand ; but in this case the lines 

 appeared to have irregularities in their ruling. A long tube 

 apochromatic dry 4 mm. Q) • 97 N.A. resolved the 90,000 band 

 ( = • 28 fi) quite easily ; the same band was also resolved by a 

 long tube dry apochromatic } • 96 N.A. with some difficulty. 



It may be remarked, in passing, that the latest books on 

 Physical Optics, used by our Universities and schools, state that 

 ao,"Voo m - ( or '28 /a) is the theoretical limit for microscopic vision ! 



A Zeiss apochromatic 12 mm. (£) for the long tube - 66 N.A. 

 resolved the 60,000 band (= *42 /a) quite strongly, but there was 

 not the slightest appearance of its resolving the 70,000 band. The 

 resolving limit of this very fine lens is probably about 65,000 

 (= '39 /a). A short tube semi-apochromatic I *84 N.A. resolved 

 the 80,000 band ( = • 32 /a), and a short tube semi-apochromatic 

 i -76 N.A. resolved the 70,000 band (= -36 /a). The bands were 

 examined with many other lenses, but nothing worthy of note was 

 observed. 



Piuled lines are more difficult to resolve than diatoms of equal 

 fineness ; we may go further, and say that ruled lines mounted in a 



