288 PROF. ZETTNOW ON A. PELLUCIDA. 



A.. 505 onwards, but of those valid on the plate only from 

 X. 505-440, as the violet were completely absorbed by the 

 realgar: in Figs. 788 and 791 a slow erythrosin plate, No. 140 

 of my emulsion, was employed, and the negative was not 

 intensified. The exposures in bright sunlight lasted two and 

 eight minutes, and would have taken about thirty minutes for 

 the 6,000 diameter picture, obtained with compensating ocular 8, 

 and, therefore, for this latter, Fig. 779, an ordinary, highly- 

 sensitive, rapid plate, No. 142 of my emulsion, was employed, 

 and exposed for four minutes. As such plates are much more 

 delicate than the less sensitive erythrosin ones, this accounts 

 for the weak character of the impression. Development with 

 pyro-gallol soda. According to my repeated measurements, in 

 which the magnification was exactly estimated, the transverse 

 striae of Amphipleura in the middle are 4,100 and the longi- 

 tudinal 5,200 per mm. 



[Note. — 1st. The exhibition of two sets of striae at right 

 angles to one another is not a proper resolution of the A. 

 Pellucida, neither are beads formed by the intersection of such 

 striae the proper resolution of the coarser diatoms. 



2ndly. The resolutionof striae and beads is no test of quality, 

 it only affords a rough measure of the aperture of an objective. 

 —Ed. " Q.M.C. Journal."] 



