PLEUROSIGMA ANGULATUM 71 



duced from theory, of what spectra of the given position and inten- 

 sity of the proposed data should give is seen in fig. 64. But what 

 seems quite as much to the purpose is, that Dr. Zeiss has produced a 

 fine photograph of P. anyulaiwm, given in Plate X., where it will 

 he seen that the details shown in fig. 64 appear. 



Let it be clearly understood that this does not pretend to be an 

 interpretation of the markings of the diatom ; it is only held by 

 Abbe to be an accurate indication by calculation of what image the 

 tfiven diffraction spectra should produce. An optical glass and 

 media for ' mounting ' and ' immersion ' of immensely greater refrac- 

 tive and dispersive indices at present wholly inaccessible to us 

 must, he contends, be found and employed before all the diffraction 

 spectra of P. angulatum could be admitted to form its absolute and 



PIG. 64. 



complete ' diffraction image ; ' but from such spectra as the objective 

 employed can admit, it is maintained by Abbe that the mathe- 

 matician can accurately show what the image will be. In the case of 

 P. angulatum theory indicated the optical, but not necessarily the 

 structural existence * of smaller markings, shown in fig. 64, between 

 the circular spots. These had not been before seen by observers ; and 

 the mathematician who made the calculation (Dr. Eichhorn) had never 

 seen the diatom under the microscope ; but when Mr. Stephenson 

 re-examined the object stopping out the central beam as above 

 described and allowing the six spectra only to pass he saw the 

 small markings, and showed them at a meeting of the Royal Micro- 

 scopical Society to many experts who were there. They were small 

 and faint, and no doubt purely optical ; and, we learn from experiment, 

 may readily escape observation ; but by careful investigation they 



1 Con/. Abbe's recent note, pp. 72 et seq. 



