96 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
image-planes is completely contained within the corresponding radial 
angle. 
(9) If a primary maximum lies in the midst of a circle of secondary 
maxima taken by pairs of opposites, and linear periodic relations between 
x and y are possible for which the expression / cos (x cos w + y sine w) 
takes a constant value, then the microscopical image in the image-planes 
assumes parallel bands of constant intensity at constant distance— in 
those directions to which the secondary maxima lie symmetrically when 
taken by pairs. 
Strehl then discusses the special cases of Synedra pulchella, Plcuro - 
sigma attenuatum, and P. angulatum. 
(10) Oblique illumination. — Here he assumes that the group of spectra 
consists of a polygonal arrangement of one primary and several secondary 
maxima. 
(11) If the primary maximum forms a circle with the secondary 
maxima, then the microscopical image along the optic axis is indepen- 
dent of the spot. 
The special cases of the same three diatoms are soverally considered. 
The subjects of astigmatism, cylindrical waves, spherical aberration, 
zonal refraction, and coma, are then similarly considered, but the results 
do not lend themselves to the enunciation of propositions. 
Fig. 10. 
New Stage Finder.* — Messrs. Koltzoff and Ivanoff propose a new 
form, which has been made for them by Reichert, of this accessory. 
* Zeitschr. f. wiss. Mikr , xv. (1898) pp. 3-7 (1 fig.). 
