288 



SUMMABY OF CUREENT RESEARCHES RELATING TO 



metre is subdivided into ten spaces of ten microns each. There are 

 thirteen of these lines at the beginning of the centimetre, the first 

 tenth of a millimetre being measured from the mean of the first three 

 to the mean of the eleventh, twelfth, and thirteenth. The scale is 

 engraved on a piece of platin-iridium made by Matthey, and containing 

 20 per cent, of iridium. 



Professor W. A. Eogers gives the results of a very elaborate 

 " study " of the scale, which is now in the custody of the American 

 Society of Microscopists, and available, under regulations, to " parties 

 of eminent ability" for the comparison and verification of their 

 standards. Three copies are to be made on glass, which will be lent out. 



Microscopic Test-Objects.* — The correspondence on this subject 

 between " Monachus " and Mr. E. M. Nelson has been further con- 

 tinued, the former finally accepting (as "that which was to be 

 demonstrated ") Mr. Nelson's admission that when he wrote that he 

 had by particular means made the discovery of the " true structure " 

 of Surirella gemma he did not mean the " ultimate true structure." 



There is one point however in the correspondence left untouched, 

 which we refer to because the misapprehension which Mr. Nelson 

 was under on the subject has at one time or another been widely 

 shared and we have no doubt is so still. 



If we have a grating (fig. 38) it will, as we know, give rise to 



Fig. 38. 



Fig. 39. 



diffraction spectra as in fig. 39 . But if we stop off all the spectra except 

 two nearest the central dioptric beam (say at the top and side) 

 we shall still see the grating. Hence it has been supposed that only 

 those spectra were really necessary for the image, or as Mr. Nelson 

 puts it, " the true structure can be seen without taking up all the 

 diffraction spectra." 



It cannot be too clearly borne in mind that this is an erroneous 

 notion and that it is a fundamental point of the diffraction theory 

 that if we are to see a true image of the object, all the diffraction 

 spectra into which the original pencils were separated must be again 

 gathered up and brought to the eye, so that wherever any of the 

 diffraction spectra (up to the limit of vanishing intensity) are wanting, 

 the image is incomplete. The absence of the spectra shut off may 

 produce very considerable variations in the image, not only in the 

 breadth of the lines and spaces, but otherwise. 



* See Bibliography, infra. 



