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XIII. — A Method of Investigating Dicttoni Striccttcre. 

 (Preliminary Communication.) 



By Hamilton Hartridge, M.A. 

 Fellow of King's College, Cambridge. 



(Read Jtme 18, 1913.) 



Heretofore the study of Diatoms has been almost limited to the 

 observation under high magnification of the images obtained with 

 various conditions of focus, aperture, and correction of the objective ; 

 this has shown the variation the image of a structure may under- 

 go, rather than determined what the structure may be which will 

 yield all the observed images. Abbe suggested a more profitable 

 line of investigation, by directing attention to the relationship 

 between the diatom and the grating. He showed that the study of 

 minute objects is intimately related to the investigation of complex 

 diffraction phenomena, by processes well known in physical optics. 

 In the present research the attempt has been made to carry that 

 suggestion to its logical conclusion, and to investigate the Diatoms 

 by the spectra they set up in the upper focal plane of the objective. 



The variation in character and thickness of the markings 

 makes it essential to be able to examine small isolated areas of 

 the valve of known size and position. For this it was found 

 necessary to employ a spectrometer of special design, since it was 

 impossible to rely on chance fractures to provide these areas. 



The instrument consisted of the following parts, which will be 

 described in turn : — 



1. Monochromatic light source. 



2. Condenser. 



3. Adjustable slits. 



4. Collimator objective system. 



5. Grating (Pleurosigma). 



6. Telescope objective system. 



7. Micrometer eyepieces. 



Light of standard wave-length was provided by a quartz 

 mercury- vapour lamp, and from it was obtained by transmission 

 through a special absorption filter the brilliant green radiation 

 (5461 A.U.). Filters were, however, provided by which either the 

 yellow (5790 and 5770 A.U.) or the violet (4359 and 4078 A.U.) 

 could be obtained instead. The lamp was enclosed in a light-tight 

 box, which was ventilated by means of a vacuum pump. In the 

 side of the box was mounted a lens, by which a parallel beam of 



