580 A. A. C. ELIOT MERLIN ON DIATOM STRUCTURE. 



illumination, but such lenses were much more expensive than those 

 made to-day of equal, or superior, optical performance. Still, 

 it is as well to bear in mind that in 1850 thoroughly well-corrected 

 dry achromatic lenses up to 0*90 N.A. were obtainable (Powell's 

 l/16th-inch of that date has the last-mentioned aperture), and 

 were capable of resolving most of the present well-known tests 

 with the exception of the A. pellucida, this being first resolved 

 (according to Dr. Carpenter) by one of Powell & Lealand's 

 water-immersion objectives which that firm commenced con- 

 structing in 1868. 



Jvurn. Qiuketl Microscopical Club, tier. 2, Vol. XII., No. 77, November 1915. 



