xiv EXPLANATION OF PLATES 



what an endothelium-cell is like, the knowledge gained of it will be small 

 indeed. 



Fig. 8 represents the same structure, x 138 diams., by an apochromatic 

 i -I'M N.A. Here only the upper surface of the pipe is seen, so that the out- 

 line of the endothelium-cells can be clearly traced. The circular elastic tissue 

 is also displayed. There is, moreover, an increased sharpness over the whole 

 picture, due to the greater aperture of the objective. 



PLATE I 



Fig. 1. The inside of a valve of Pleurosigma angulaturn, showing a 

 ' postage-stamp ' fracture, x 1750 diams., with an apochromatic ^ 1-4 N.A. by 

 Mr. T. F. Smith, and illustrating his view of the nature of the Pleurosigma 

 valve. 



Fig. 2. The outside of a valve of Pleurosigma angulaturn, showing a dif- 

 ferent form of structure, x 1750 diams., with an apochromatic 1*4 N.A. by 

 Mr. T. F. Smith. These two photo-micrographs demonstrate the existence of at 

 least two layers in the angulatum. 



Fig. 3. Coscinodiscus asteromphalus, x 110 diams., with an apochromatic 

 1-inch -3 N.A. 



Fig. 4. A portion of the preceding, x 2000 diams., to show the lacework 

 inside the areolations. This lacework is believed to be a perforated structure, 

 as a fracture passes through the markings. In the central areolation there 

 are forty-six smaller perforations surrounded by a crown of fifteen larger ones. 1 

 Photographed with an apochromatic l - 4 N.A. 



Fig. 5. Aulacodiscus Kittonii, x 270, by an apochromatic 1-inch -3 N.A. 



Fig. 6. A small portion in the centre of an Aulacodiscus Sturtii, x 2000, 

 by an apochromatic 1-4 N.A. Broadly speaking, the difference between the 

 Coscinodisci and the Aulacodisci lies in the fact that in the former the 

 secondary structure is inside the primary, while in the latter it is exterior to it. 

 This definition, however, is not strictly accurate, as it is believed that the fine 

 perforated structure covers the entire valve, it being only optically hidden by 

 the primary structure. 



The whole of these demonstrations were photographed for the present 

 Editor by his friend E. M. Nelson, Esq., and have been reproduced from the 

 negatives by a process of photo printing. 



PLATE II. (Facing p. 274) 



ARRANGEMENT OF THE MICROSCOPE WITH A STAND FOR THE MICROMETER 

 EYE-PIECE, TO SECURE STEADINESS AND ACCURACY IN MEASUREMENT 



PLATE III. (Facing p. 286) 



ARRANGEMENT OF THE MICROSCOPE AND ACCESSORIES FOR THE EMPLOY- 

 MENT OF THK CAMERA LUCIDA 



PLATE IV. (Facing p. 334) 

 THE METHOD OF USING THE SILVER SIDE REFLECTOR OR PARABOLOID 



PLATE V. (Facing p. 410) 



METHOD OF USING DIRECT TRANSMITTED LIGHT WITHOUT THE 

 EMPLOYMENT OF THE MIRROR 



PLATES II. to V. are engraved from photographs, taken at the request of 

 the Editor by Mr. E. M. Nelson, from the arranged instruments. 



1 A section of this diatom will be found in the Transactions of the Count// of 

 Middlesex Natural Hi^loi-ij Society for 1889, Plate I. fig. '2. 



