Notes. 395 



tried, and the sun having obligingly shown himself steadily for a 

 few hours, I am able to give some fresh details, which may be of 

 interest. 



On one of these plates, under these new conditions, I saw 

 the 120,000 (-21 fi) band clearly resolved, with the cheap oil- 

 immersion y 1 ^ (long tube) of 1*3 N.A., and the 4 eye-piece. 

 Previously I had failed to see this, although a friend had done 

 so. This band was also resolved by a Powell and Lealand water- 

 immersion -j^ N.A. 1 • 2, and a 5 eye-piece. 



The semi-apochromatic \ N.A. ■ 76 resolved the 80,000 (-32 p) 

 band ; some years ago attention was directed to the remarkable 

 resolving power of this objective. 



The 90,000 ('28 p) band was resolved by a Powell and 

 Lealand apochromatic dry J, with the 12 eye-piece. 



While the instrument and heliostat were in position, it occurred 

 to me to try the dry front of the Powell and Lealand water- 

 immersion y^, when, to my astonishment, the 100,000 ("25 /*) 

 band was plainly and well resolved. 



This lens is one of the series that used to be known as Powell's 

 new formula water immersions. It was introduced by that firm in 

 December 1874; the I appeared first, and became very celebrated ; 

 afterwards a y*g was produced, the y 1 ^ came later, probably about 

 1877. These lenses were the finest extant before the advent of the 

 oil immersions. 



Being so successful with this dry y^th, I next tried two old 

 y^-ths, one by A. Ross, N.A. *81, and the other by Powell, N. A. '93, 

 both being constructed upon Lister's formula, with triple fronts 

 and backs, and doublet middles. 



The A. Eoss showed the 80,000 (*32 /n) band very strongly, and 

 the 90,000 ( ■ 28 /*) as an exceedingly difficult image. 



The Powell did the 90,000 band quite easily, and, strange as it 

 may appear, the 100,000 band ("25 fi) was certainly resolved, 

 although the image was a difficult one. 



It was a great surprise to find a Microscope objective, made 

 more than half-a-century ago, resolving a 100,000 (*25 //,) band. 



In all cases oblique light in one azimuth was used from an 

 achromatic oil-immersion condenser. 



It is clear, therefore, that with this new screen the limit of 

 100,000 times the N.A. of the objective (mentioned in my previous 

 Note) is fully established for practical work. 



Mr. Grayson having placed test band-plates, properly ruled and 

 properly mounted, within our reach, it remains for microscopists 

 to make the best use they can of them, and to endeavour that their 

 practical work shall lag behind the theoretical limit as little as 

 possible. 



