Invcstiyatiny Diatom Structure. Bij H. Hartridye. 367 



Now, as mentioned above, the isolation of these areas was 

 carried out optically, by suitable apparatus applied to the observ- 

 ing telescope ; this will now be described. Above an immersion 

 semi-apocliromatic objective was mounted a convergent lens, and 

 by these a highly magnified image of the diatom was focused on 

 to a slip of stained gelatin film. This carried at its centre a fine 

 rectangular perforation, and was mounted on an adjustable earner. 

 The dye with which the gelatin was stained conferred on it the 

 property of being opaque to the green mercury radiation, without 

 affecting its transparency to the yellow or violet. By this means 

 it was possible, by a micrometer eye-piece, exactly to locate the 

 portion of the diatom corresponding to the perforation, using the 

 yellow and ^dolet rays, and then immediately afterwards by simply 

 cutting off these rays with a filter to render the screen opaque, 

 and only to allow therefore those pencils to be transmitted which 

 correspond to the perforation. A few centimetres above the 

 perforation a magnified image of the upper focal plane of the 

 objective was formed, together with the diffracted pencils set up 

 by the area of the diatom corresponding to the perforation ; these 

 images could be investigated by a special micrometer eye-piece. 

 The exact technique depended on the measurements being made, 

 and will be separately considered for each case. 



The points on which evidence was required were as follows : — 



1. The distance between neighbouring structural elements. 



2. The an^le which a series of these elements make with one 

 another. 



3. The contour of the valve as seen in section. 



4. The thickness of the siliceous envelope. 



5. The character of the markings. 



6. The evidence of laminar structure. 



The method employed for making each of the above measure- 

 ments will now be described. Nos. 1 and 2 were carried out 

 simultaneously as follows : — 



In the slit-holder was placed a disk of brass bearing two small 

 rectangular apertures. These were situated 8 mm. apart, on oppo- 

 site sides of the optical axis. Above each was mounted a small 

 glass-plate micrometer, by which the image could be made to shift 

 by small amounts. But the axes of the turn-tables to which the 

 glass plates were fixed were at right angles to one another, so that 

 while one controlled the distance apart of the images, the other 

 changed the angle which they made with an object lying on the 

 stage. The light from these slits was rendered parallel by the 

 collimator and then fell on the diatom, by which a number of 

 diffracted pencils were formed. By slightly rotating the slits, two 

 of these pencils could be made to lie side by side in the centre of 

 the upper focal plane of the objective. And by now adjusting 

 the two glass-plate micrometers, the diffracted images of the slits 



