520 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



illuminant. The projecting mirror (fig. 86), about 4x3 in. in size, 

 is fixed to a metal arm, about 4 J in. long, which is connected with 

 the draw-tube of the Microscope by means of a clamping ring. It is 

 fixed so as to incline towards the face of the ocular at an angle of 45°, 

 and can be adjusted for length by sliding the arm in or out. 



Fig. 87. 



The screen (fig. 87), the shape and size of which, and also the 

 position of the ocular and mirror, may be gathered from the illustration, 

 is constructed of cardboard or thin wood. 



(4) Photomicrography. 



Resolution of Edges in Microscopical Images.* — In this article, 

 H. Siedentopf discusses the resolution of objects which are ultramicro- 

 scopic in two dimensions only. Such objects are frequently exemplified 

 in the case of free edges, bacteria, scratches, etc. The edges produce 

 ultramicrons of the nature of diffraction -stripes rather than of diffrac- 

 tion-disks, and are best seen by dark-ground illumination. The 

 breadth of the stripe varies as the aperture of illumination and as the 

 wave-length. But, while with ultramicrons of approximately equal 

 diameter the direction of light-concentration is immaterial, it is al- 

 together another matter with edges, which are necessarily best revealed 

 with light of a suitable inclination. Such objects are, therefore, closely 

 dependent upon the azimuth of illumination. The author explains this 

 term by supposing the Microscope tube vertical and by considering a ray 

 which strikes the tube-axis at a certain angle. This ray and the axis are 

 in a certain plane, and if the observer on looking down the tube 

 imagines a clock-dial in front of him, the position of this plane can be 

 fixed by reference to the plane containing the tube-axis and the figures 

 xii and vi on the dial. Thus an azimuth of 00° would lie to the due 

 right of the observer. The azimuth of an edge in a microscopical 



* Zeitschr. wiss. Mikrosk., xxv. (1908) pp. 424-31 (1 pi. and 2 figs.). 



