SOME PROGKESSIVE EXAMPLES 803 



the photographs taken in varying positions so that the incidence 

 of the light was changed in each case. 



Fig. 2, the light was thrown in the direction of the length 

 of the frustule, bringing out transverse lines. 



Fig. 3, the light was in the direction of the width of the 

 frustule, bringing out longitudinal lines. 



Fig. 4, the incidence was between the two positions of Figs. 

 2 and 3, bringing out the dotted structure. 



Fig. 5 was taken with a central solid cone of light, but with 

 reduced aperture. 



PLATE IX, FIG. 6. Amphipleura pellucida. X 1500. 



Exactly the same conditions of illumination obtained as in 

 Fig. 1, except that the sub-stage iris-diaphragm was rotated 

 so that instead of the illumination being in the direction of the 

 length of the frustule, it is approximately at an angle of 45 to 

 it. Very few frustules will show the dotted image at all, and 

 it is only in exceptional specimens that it is possible to bring it 

 out. The one chosen should be as large as possible, and as a 

 rule it will be found that if the specimen is such that lines 

 can be clearly shown in the direction of the length of the 

 frustule by illuminating at right angles to the length, the 

 dots may possibly be seen by suitably varying the incidence of 

 the light. Perhaps the best method is to set up the object on 

 a microscope with a rotating stage, arrange the illumination 

 so that either transverse or longitudinal lines are shown, and 

 then to rotate the stage carefully until the dotted image is seen 

 with more or less distinctness. 



PLATE X, FIG. 1. Surface of Metallic Alloy Tin and 

 Antimony, x 20. 



Three-inch achromatic objective, by James Swift & Son. 

 No ocular. 



Illuminant, electric arc, taking 4 amperes. 

 Convergent beam projected on to the specimen at an angle 

 of 45 by means of ordinary bull's-eye condenser. 

 Gifford F-line screen. 

 Plate, Imperial orthochromatic. 

 Exposure, 45 seconds. 

 Development by metol-hydrokinone. 



