SCALE OF PONTIA BRASSICAE

SLIDE: 19th century slide

Scales of the underside of the wing of Pontia brassicae have been used as a test object since the earliest such use. The longitudinal striae appear like faint dashed lines with low power objectives. The the beaded appearance of these becomes visible with a 20X objective with a n.a. of 0.46, especially with oblique lighting, and it is this threshold that was used to 20X objectives with about this n.a. The beading becomes very distinct at 40X with an n.a. of 0.70