S4 



THE DEPOSITS OF THE SEA-BOTTOM. 



be drawn on the diagram without being too marked, what is also the ease with some very fine radia- 

 ting lines, sometimes found on the marginal plate. 



We shall get the best idea of the refraction of the Coccolith by observing the light shadows 

 which are seen when the tube is moved up and down, and which here give very characteristic figures. 

 The common rule is that at the boundary between two media of different refraction a whitish shadow 

 will be formed over the medium with the stronger refraction, if the field of view be above the object, 

 while it will be seen over the medium with the slighter refraction, if the field of view be under the 

 object By moving the tube up and down we shall, when looking at a Coccolith, get a system of 

 luminous rings, which may be elucidated by the following figure. 



The Coccolith is seen in a transverse section, and the rays likewise are imagined transverse 

 sections of the ringformed systems seen by different adjustments. If the field of view is immediately 

 over the Coccolith (line a) a strongly luminous, elliptic ring will be seen over the central ring, and 

 another, much fainter ring outside of this, almost at the outer edge of the marginal plate. If the 

 tube is lifted they will approach more and more, until at b they will pass into one ring; farther up 

 the two rings will pass through each other, and the formerly outer one will soon gather to a very 

 faint and vague spot, still, however, to be distinguished from the luminous ring round it (transverse 

 section c); the shadows, of course, become more and more faint, and cover larger areas, the farther 

 they go from the starting-point. The highest figure we get, is a very faint circle of light (line d)\ 

 the rays from the middle of c now have passed through each other and joined the outer circle. If 

 the tube is lowered below the preparation, the first thing seen (line e) will be a very faintly luminous 



