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16. Values of the differential quotients used in the preceding 
articles. 
The following formulae may serve for the various differential 
quotients used in the preceding equations. 
(For the meaning of the letters see fig. 1). 
0%  _—-cos D cos O 
Are. sin À 
Ox PANE Ò sin X 
9D eos Dina 
Mis ro 
rhe cos O sin ¥ 
oA 
ID =— — tds O 
where %, 4 and O are to be computed by 
sin À sin y = sin (a — A) cos D 
sin À cos ¥ = cos (a — A) cos D sin 0 — sin D cos 0 
sin À sin O = sin (a — A) cos 0 
sin À cos O = — cos (a — A) cos 0 sin D + sin Ò cos D. 
A few observations of Prof. Jan DE Vries and Prof. J. A. C. 
OUDEMANS were answered by the lecturer. 
Mathematics. — “On twisted quintics of genus unity.” By Prof. 
JAN DE VRIES. 
1. By central projection a twisted curve of order five and genus 
unity can be transformed into a plane curve of order five with five 
nedes. Consequently in each point of space meet five chords or 
bisecants of the twisted curve Rs. 
If the centre of projection is taken on A; a curve of order four 
with two nodes is obtained. From this ensues that through each 
point of A; two trisecants may pass. 
2. The bisecants that meet a given right line / form a surface 
