124 Dr. G. C. Wallich on the Markings of the Diatomacea 



unaided eye, and demanding the highest of our optical aids to 

 render its nature manifest. These glasses have certainly met 

 with little encouragement in this country ; but this depends on 

 no fundamental error either in M. Nobert's principle or his 

 handiwork, but on a minor defect, which will, without doubt, be 

 speedily overcome. I allude to the difficulty of engraving the 

 different series of hues on slips of glass sufficiently thin to admit 

 of their employment under the highest and most delicately ad- 

 justed combinations of the microscope. It is universally allowed 

 that the accuracy of the lineation cannot be surpassed, the last 

 and closest series of lines being as regular and distinctly ruled 

 as the first or most distant. The difficulty can hardly be over- 

 come, however, until a separate slip of glass is devoted to each 

 two of the first five or six series, and each one of the remaining 

 number. This would, of course, enhance the cost to a consider- 

 able extent; but a truly effective standard would be the result, 

 and we should establish a check upon the fallacious test-objects 

 hitherto resorted to. It must be evident that the more perfect 

 our instruments become, the more urgent is the requirement for 

 an undeviating standard of comparison. When this is known to 

 exist, purchasers of lenses will be provided with a safeguard ; 

 and, as a necessary consequence, the efforts to produce improved 

 apparatus will be redoubled. 



The subjoined Table gives the lineation of some of the com- 

 monly employed test-species, according to the authorities noted 

 in the first column. M. Sollitt^s measurements, it will be seen, 

 embrace the extreme ranges. The accuracy of his figures is 

 unquestioned, as regards all but the two last-named forms on 

 the list ; and in these it has met with scepticism solely from 



Tabular Statement of Lineation in 1000/A of an inch. 



* ' The Jlioroscope,' by Prof. Carpenter, p. 205. 



t Quarterly Journal Microscop. Science, No. 29, p. 51. 



