( 1088 ) 
(B,, 8.) and s describes a regulus =. To B, corresponds the point 
at infinity of B,P,, to B, that of B,P,, so that in = are lying the 
rays through B,//B,P, and through B,//6,P,. From this ensues 
that = touches 9, and 8, in B, and in B, so that Oz is an axis 
of symmetry of 2 and O the centre. 
So a congruence of revolution (2,2) is generated by the revolution 
of a regulus about one of its axes of symmetry. 
If only real congruences are taken into consideration, the two kinds 
can be distinguished : 
1. The regulus revolves around an axis of symmetry not cutting 
it; then the focal surfaces are one-sheeted hyperboloids of revolution. 
2. The regulus revolves around an axis of symmetry cutting it; 
then one of the focal surfaces is an ellipsoid of revolution, the other 
a two-sheeted hyperboloid of revolution. 
In the former case the contact of /’? and #,* and the regulus 
= takes place in imaginary points, in the latter case in real points. 
In both cases the sections of „Oy with F? and F,’ consist of 
circles having as radii the axes of symmetry of = lying in zy. 
One of them is imaginary in the second case. 
ry 
Physics. — “Observations concerning anomalous dispersion of light 
in gases.” (First communication). By Prof. W. H. Junius and 
B. J. VAN DER PLAATS. 
Although it be now generally admitted, that anomalous dispersion 
of light must influence certain astrophysical phenomena in some 
degree, yet the majority of astrophysicists opine, that such influence 
cannot bear a general or radical character, but may in some special 
cases only, near a few lines of the spectra of celestial bodies, per- 
haps beeome apparent. 
In order to make out whether that opinion is tenable, one has 
to answer two questions. First: Is anomalous dispersion an excep- 
tioual or a general phenomenon, appearing in the vicinity of every 
absorption line — provided that the conditions of the observation 
are properly chosen? And secondly : Does the present state of physical 
and astrophysical knowledge make it very probable or not, that in 
the atmospheres of celestial bodies conditions prevail, from which 
the appearance of quite conspicuous effects of anomalous dispersion 
necessarily follows? 
In this paper we are not going to consider the second question ; 
