E. M. NELSON ON AMPHIPLEURA LINDHEIMERI. 315 



AMPHIPLEURA LINDHEIMERI. 



By Edward M. Nelson, F.R.M.S. 



(itearf December 23rd, 1913.) 



Half a century ago Xavicula rhomboides was the accredited test 

 for the best microscope lenses. This was the common " English " 

 rhomboides, which has about 72,000 to 73,000 striae per inch ; it 

 was also known as the Amician test. About the seventies 

 iV. rhomboides was discovered in America. This was a coarser 

 form, having some 60,000 striae per inch, consequently any 90 

 | inch N.A. 0*71 would resolve it readily. In those days there 

 were no cheap apertometers to be had, so testing an objective 

 merely meant a measurement of aperture by resolving striae 

 on some diatom by means of oblique light in one azimuth. We 

 now know that the feat can be accomplished by a very badly 

 corrected objective. 



The new coarse American rhomboides became very popular, and 

 diatom dotters and brassey glassites simply revelled in it. 



History has, however, repeated itself, for as time went on lenses 

 improved, and both the coarse and fine rhomboides failed as tests 

 for high powers, so others had to be found to fill their place. 

 Amphipleura pellucida became the test for immersions, while 

 A. Lindheimeri was used for dry lenses. As A. Lindheimeri has 

 about 7 ",000 striae per inch, it is a very suitable test, with oblique 

 light from a dry condenser, for lenses of the 7a type. 



This w T as the favourite test of the late Lewis Wright, who 

 mentions it in his excellent book on the Microscope. But now 

 another Lindheimeri has been discovered in Spain, and as it is a 

 coarser variety, it is necessary to distinguish between these forms 

 when quoting the Lindheimeri as a test. The new Lindheimeri 

 has 67,000 striae per inch, and therefoi e is easier to resolve than 

 the old English rhomboides ; a | inch, or 8 mm., will very nearly 

 resolve it in fact, they do so in patches; a Powell 100 | inch 

 of 1S75, which would fail on an English rhomboides, resolves it 

 easily. 



The new Lindheimeri can be recognised at once by its very long 

 terminal nodules, the terminal nodule being one-third of the whole 

 length of the valve, while in the old form it is only one-fifth. 



