170 THE MICROSCOPE AND ITS REVELATIONS. 



Aperture. On the nineteen-band Test- plate the lines are ruled at the 

 following distances, expressed in parts of a Paris line, which, to an Eng- 

 lish inch, is usually reckoned as .088 to 1.000, or as 11 to 125: 

 Band 1. l-1000th. Band 8. l-4500th. Band 14. l-7500th. 



2. l-1500th. " 9. l-5000th. " 15. l-8000th. 



3. l-2000th. " 10. l-5500th. " 16. l-8500th. 



4. 1.2500th. " 11. l-6000th. " 17. l-9000th. 



5. 1.3000th. " 12. l-6500th. " 18. l-9500th. 



6. 1.3500th. " 13. l-7000th. " 19. 1-1 0000th. 

 " 7. l-4000th. 



The following exact estimates of the numbers of the lines to the Eng- 

 lish inch, in some of the Bands, are given by Dr. Royston Pigott: 1 



I. 11,259.51358. IX. 56,297.56790. XV. 90,076.10864. 



III. 22,519.02716. XL 67,557.08148. XVII. 101,335.62222. 



IV. 33,778.54074. XIII. 78,816.59506. XIX. 112,595.13580. 

 VII. 45,038.05432. 



In objects likeNobert's Test-plate, spurious diffraction lines are easily 

 mistaken for genuine resolution; and the difficulty of resolving the higher 

 bands of his series was formerly supposed to be an optical impossibility. 

 The more recent investigations of Helmholtz and Abbe, however, have 

 'disposed of this theoretical objection; and the 'resolution ' of Nobert's 19th 

 band, which was long supposed to be a sort of crux of Microscopy, is 

 now easily demonstrable. 



161. It cannot be questioned that the recognition of the value of the 

 markings on the siliceous valves of the Diatoms as Test-objects (first 

 made by Messrs. Harrison and Sollitt, of Hull, in 1841) has largely con- 

 tributed to the success of the endeavors which have since been so effectu- 

 ally made, to perfect high-power Objectives, and to devise new methods 

 of using them to the best advantage. But it has now been demonstrated, 

 both theoretically and practically, that the power of ' resolving' these 

 markings essentially depends on the Angular aperture of the Objective: 

 so that, as a lens which possesses it in a high degree may be very defi- 

 cient in 'definition,' and will probably have an inconveniently short 

 f working distance' with very little ''penetration' qualities essential to 

 an Objective to be employed in Biological investigation, the resolution 

 of difficult Diatom-tests by no means proves the fitness of an Objective 

 for the ordinary work of the Microscopist. Still, these tests are of great 

 value for the purpose to which they are really adapted; and it will there- 

 fore be desirable here to specify their relative degrees of ' difficulty, * 

 which is indicated by the closeness of their lineation, leaving for future 

 discussion ( 277) the nature of the structure to which that lineation is 

 due. The greater part of the Diatoms now in use for this purpose, are 

 comprehended in the genus Pleurosigma of Prof. W. Smith; which 

 includes those Naviculm whose 'frustules' are distinguished by their 

 sigmoid (S-like) curvature (Fig. 165). 



1 "Monthly Microscopical Journal," Vol. ix. (1878), p. 63. A much larger 

 number of lines to the inch has been assigned to Nobert's Test-plate by Mr. J. 

 Allan Broun ("Proceedings of Royal Society," Vol. xxiii., 1875, p. 531), on the 

 basis of his measurement of Photographs taken by Dr. E. Carter (Surgeon U. S. 

 Army); but there seems strong ground to believe that either from diffraction, 

 or from some mistake in the magnifying power employed, Mr. Broun's estimate 

 must be greatly in excess of the reality. 



