ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 521 



preparation can be similarly stated and the relative azimuth of such an 

 edge to the illumination would he the difference of edge-azimuth and 

 illumination-azimuth. As a good working principle, the author lays it 

 down that considerable quantities of light are diffracted by edges and 

 such-like objects in a direction perpendicular to themselves (i.e. in the 

 direction of the Microscope axis) only if the illumination-azimuth is 

 approximately perpendicular to the plane which contains the edge and 

 the Microscope axis. 



The appearance is easily studied in the back focal plane of the 

 Microscope objective after extraction of the ocular. It is seen with 

 both transparent and absorbent bodies and with both microscopic and 

 ultramicroscopic edge-thicknesses. Among objects with ultramicroscopic 

 edges must be reckoned many bacteria as well as very fine crystalline 

 needles. These last would essentially decompose the light in the 

 neighbourhood of an edge, which might perhaps act as a very sharp- 

 pointed prism. In order therefore to make edges visible in the micro- 

 scopical image, it is requisite that the illumination lie in relative 

 azimuth of 90° or 270° to them. If it is desired that at any selected 

 azimuth all edges in the object should appear in the image, then the 

 illuminating rays must impinge at all azimuths, and 

 all azimuths of the illuminated condenser aperture 

 must be filled with light. But if the azimuth of 

 illumination be limited by a slit-diaphragm (fig. 88) 

 placed in the Abbe diaphragm holder, then in the 

 microscopical image only those edges appear whose 

 azimuth relative to the slit is 90° or 270°. On rota- 

 tion of the slit-diaphragm a continuous alteration of 

 the image will result, some edges disappearing and 

 others coming into view. 



The author illustrates his treatise by a plate showing the images of 

 some plankton material from the Mediterranean. One view is due to 

 light in all azimuths and two views of the same object to light at 

 azimuths of 90° to each other. The views, at first sight, would seem 

 absolutely discordant. 



If the condenser is racked a little out of focus, the centre of the 

 field appears dark and the diffraction-stripes group themselves in a 

 merry-go-round fashion (Karusselformig), and as the slit-disk is rotated 

 these continually change, only those edges coming into view which are 

 perpendicular to the azimuth of illumination. The author suggests 

 that an application of his ideas might be of practical value in helping 

 to bring out special detail in an object. As another illustration he 

 shows the very different appearances presented by SjrirocJtseta pallida 

 Tinder similar treatment. 



Simple Stand for Photography of Macro- and Microscopic Objects.* 

 H. L. Heusner has found that he can get a very useful stand out of a 

 cylindrical iron rod of 20-25 mm. diameter and 100-150 c.cm. length, 

 (fig. 89). The lower end fits into a heavy foot and rests upon the floor, 

 or on a stove, and the upper end is arranged for convenient clamping to 



* Zeitschr. wiss. Hikrttsk., xxv. (190S) pp. 432-3 (1 fig.). 



