474 MANIPULATION. 



It will be noticed that the so-called longitudinal and trans- 

 verse strise are resolved into dots, which are so arranged as to 

 present under object-glasses of low power the appearance of 

 longitudinal and transverse lines. When viewed by a power 

 of 500 diameters, it is readily seen that its surface is convex, 

 and that the dots are projections from the surface ; a curved 

 structureless line runs down the middle of the shell in the 

 centre, and at each end the line is expanded into an oval spot ; 

 on the edges, near the central spot, the dots are elongated 

 transversely, and appear as so many short bands. This species 

 of navicula is an excellent test for a quarter of an inch object- 

 glass, which should show distinctly both sets of lines or dots 

 by oblique illumination. 



Navicula Angulata. — This exceedingly beautiful species was 

 first found upon conferva in the Humber at Hull, and three 

 sets of lines discovered on it, by some microscopists residing 

 there. Since then, its structure has been carefully worked 

 out by Mr. Gillett by an approved method of illumination 

 and of mounting between thin glass, and these supposed 

 lines have been resolved by him into minute dots or eleva- 

 tions from the surface, which are so arranged as to present 

 longitudinal, transverse, and obHque markings, under certain 

 conditions of illumination. Fig. 4, plate ix., is an entire 

 specimen seen under a magnifying power of 500 diameters; 

 and fig. 5, a portion of the same magnified 1,200 diameters; 

 whilst at fig. 6 is represented a still more highly magnified 

 view of a portion of another specimen, for which the author 

 is indebted to Mr. Gillett, from whose microscope it was 

 sketched by Mr. Leonard ; the angle of inclination of the dots 

 to the sides of the shell was found on measurement to be 51° 

 in some specimens, and nearly 60" in others. Whenever 

 these infusoria are viewed by means of very oblique light, 

 the appearances presented are those shown in figs. 4, 5, 6 ; 

 but under the most favourable illumination, either from a 

 white cloud, or a lamp with direct light, and a magnifying 

 power of at least 1,200 diameters, the lines are all shown to 

 be dots or elevations from the surface, being exhibited as they 

 occur on a small portion of the shell by fig. 7. Figs. 1, 2, 4, 5, 



