88 HENDRY, ON AMPHirLEURA PELLUCIDA. 



objective to different foci; thus the index being marked 5, 

 10, 15, 20, the revolution of index occasions circumstances of 

 importance in actual practice. 



It is hence evident that, with index at 10, the micrometer 

 value may be either 17,500ths or 16,000ths of an inch, and so 

 also of any other term of indices ; and hence the necessity, 

 in every case of measurement, that the objective should be 

 removed from the body of the instrument, and carefully ex- 

 amined as to the relations of the index to being wholly or 

 partly covered or uncovered, when differences will be thus 

 obtained materially affecting the value of observations, espe- 

 cially in dealing with fine striation. 



In slide 9 the lines are not fully developed, or rather dis- 

 played, but exhibit tops and bottoms, id est, marginal mark- 

 ings so regular as to leave not a shadow of doubt of their 

 being of the true nature of sti'ise ; in no one instance have I 

 ever been able to resolve a coarse striation of Amp. pelliicida 

 into dots, like angulatum, &c. I suppose such transverse 

 striae to partake of the character of canaliculi. 



Slide 1 1 exhibits a singular development of lines, the dark 

 colour of which, together Avith the intermediate spaces of 

 light, surpass any diatom I have hitherto seen; and thus 

 leaving not a shade of doubt as to truthfid interpretation by 

 the most sceptical regarding the existence of veritable stria- 

 tion. 



I believe the severity of test to depend not so much on the 

 number of lines in 'OOl', as on degree of development. For 

 example, compare Nobert^s test-plate, 14th and 15th bands, 

 with the Pasciola; and both these, again, with the lines upon 

 Nitzschia angidaris ; all of which range from about 52 to 56 

 in "001'', but differ widely in facility of resolution. Neither do 

 the highest powers invariably exhibit the highest markings 

 equally distinct with powers somewhat inferior ; penetration 

 being required in some cases even at a sacrifice of amplification. 

 Observe Nitzschia sigmoidea, for example, with ^th and -j^^h 

 objectives. So also with Amphiplewa pellucida ; and although 



