FRUSTULIA RHOMBOIDES:

Frustulia rhomboides was also called Navicula rhomboides and also Vanheurkii rhomboides. The longitudinal striae can be barely discerned with oblique 40x illumination. NIC at 40X with n.a. of 0.70 can also just barely resolve the areolae with careful focusing and aperture adjustment. It requires oil immersion with a high n.a. to resolve all the areaolae well. The sharpness of the areolae can be used to compare the quality of high power objectives. Because of the small areolae, it is not a good test for low or medium n.a. objectives.

Frustulia rhomboides 40X objective oblique illumination:

Frustulia rhomboides 100X objective Oblique Illumination: