Boyal Microscopical Society. 161 



Harrison and SoUitt, and were estimated by them at from 120,000 

 to 130,000 to the inch. These measurements have been adopted in 

 the ' Micrographic Dictionary,' and by most of the Enghsh writers. 

 Dr. Carpenter and Dr. Pigott are exceptions. Dr. Carpenter has 

 "not been able to satisfy himself, however, of the correctness of 

 Mr. SolHtt's estimate of the distance of the striae, and is still disposed 

 to regard it as too high."* Dr. Pigott on the other hand makes 

 them as fine as 150,000 to the inch. 



Of the American observers, IMessrs, Sullivant and Wormley 

 failed to make out the striae, but this was ten years ago.f 



A few months since I received from Messrs. Powell and Lealand 

 a special stage with a small hemispherical condenser, contrived for 

 the purpose of showing the lines on this test. I ultimately found, 

 however, that a pencil of monochromatic light converged by an 

 achromatic condenser of high angle gave better results, provided the 

 condenser was thrown somewhat out of the optical axis of the instru- 

 ment so as to secure sufficient obliquity. With such illumination, 

 the immersion x^-th of Powell and Lealand resolved the smallest 

 frustules ; the immersion ^th of the same makers resolved the coarser 

 ones, but was not adequate to resolve the finer. 



The best mounted specimens at my disposal were furnished by 

 Messrs. Powell and Lealand a few months since. They wrote me 

 they were not able to obtain cleaner samples at the time. The 

 frustules are dry and are adherent to the lower surface of the thin 

 glass cover, which was disfigured by fine debris, as is only too 

 plainly shown in the photographs, though these were taken from 

 the cleanest portion of the slides. The photographs show the trans- 

 verse markings on the frustules as fine, somewhat beaded, striae. 

 The analogy of other diatoms justifies the presumption that Dr. 

 Pigott is right in speaking of these striae as rows of beads. I was 

 not able, however, to make them out as separate beads on the 

 specimens at my disposal. Whenever I can procure some clean 

 frustules, mounted on a thin cover and with thin underglass, I will 

 try what I can do further. Meantime I should be glad to learn 

 how nearly the appearances I have photographed correspond with 

 what is actually seen by those English observers who are most 

 familiar with this test. 



On one point, however, I have no doubt, namely, the distance of 

 the beaded striae from centre to centre. As Dr. Carpenter correctly 

 thinks, this has been set down at too small a figure. In fact, there 

 exists in many quarters a disposition to underrate the actual size of 

 many minute objects, which is to my mind a convincing proof of 

 the very imperfect manner in which these must have been seen. 

 For measurement, with at least approximate accuracy, offers no 



* ' The Microscope and its Eevelations,' 4th edition, p. 182, note. 

 t * American Journal of Science and Arts,' vol. xxxi., p. 12, 1861. 



