This illustration well emphasises the difficulty there is in determining structure under precisely similar conditions to those we are accustomed to of examining valves of diatoms under the microscope. If these photographs be held in front of a strong light, they at once convey different impressions to the mind, the hemispheres appearing depressions in the one, and raised beads in the other. Both are prints from the same negative, but in mounting are reversed; and therefore the apparent dissimilarity is due to a slight inequality of illumination, which the mind accepts as light and shade. Very similar appearances to those described will result if a thin plate of glass were studded with minute, equal, and equi-distant plano-convex lenses, the foci of which would very nearly lie in the same plane. If the focal surface, or plane of vision, of the objective be made to coincide with this plane, a series of bright points will result, from the excess of light falling on each lens. If the plane of vision be next made to coincide with the surfaces of the lenses, these points would appear dark, in consequence of the rays being refracted towards points _now_ out of focus. Lastly, if the plane of vision be made to coincide with the plane _beneath_ the lenses that contain their several foci, so that each lens may be, as it were, combined with the object-glass, then a second series of bright points will result from the accumulation of the rays transmitted at those points. Moreover, as all rays capable of entering the objective are concerned in the formation of the second series of bright focal points, the first series being formed by the rays of a cone of light only, it is evident that the circle of least confusion must be much less, and therefore the bright points better defined in the first than in the last series.