TRANSACTIONS OF SECTION A. 731 
from the limb really come from a considerable depth, where the velocity of 
rotation is less. The observations above given do not support this view, since the 
velocity deduced from them is a little too great, It is altogether probable that 
the discrepancies noted by Bélopolsky are due to errors of observation. If, how- 
ever, the slit were not properly placed, a velocity slightly too small would be 
obtained, since the angular velocity of the surface diminishes with increasing 
' latitude, and falls off quite rapidly in the region near the equator. 
In 1895 the author succeeded in showing, by an extension of the same method, 
that the velocity at any point on the ring of Saturn is that of a particle moving 
in obedience to Kepler’s third law, and hence that the ring is not a solid body. 
Attempts to determine the rotation of Venus have so far been unsuccessful, 
2. On the Photo-electric Sensitisation of Salts by Cathodic Rays. 
By Professor J. Exster and Professor GEITEL, Wolfenbiittel, Germany. 
The results of the investigation made by the authors may be summed up as 
follows :— 
Cathodic rays falling upon the chlorides of czesium, rubidium, potassium, sodium, 
lithium, clear fluorspar, and even powdered glass, convert these salts into 
substances which are incapable of retaining a negative charge of electricity when 
exposed to light belonging to the visible part of the spectrum. 
All circumstances capable of abolishing the colours produced by cathodic 
radiation also destroy the photo-electric sensitiveness. 
A complete account of the investigation will shortly be published in Wiede- 
mann’s ‘ Annalen.’ 
3. On Certain Photographic Effects. By Professor P. pe HrEn. 
4, Some Experiments on Absorption and Fluorescence. 
By Joun Burke, B.A. 
Fluorescent bodies are generally more or less transparent to the rays they emit. 
The experiments were with a view to detecting whether any difference exists 
between the absorption when a body is fluorescing and when not. The comparisons 
were made with a form of double slit photometer in which photography was 
employed, described at length in the paper. Allowing for the various sources of 
error which may possibly arise, there still remains a marked difference between the 
iutensity of the light transmitted in the two cases, amounting in some instances 
to a difference of 40 per cent. in the absorptive power. Thus a substance such as 
uranium glass would appear to be less transparent to the yellow rays from a candle 
in daylight than in the dark. The latter part of the paper deals with the influence 
of dissociation on fluorescence and with the theory of fluorescence itself. 
5. On Homogeneous Structures and the Symmetrical Partitioning of them, 
with application to Crystals.! By WittiaM Bartow. 
This paper 1s the outcome of several years’ study of the geometrical possibilities 
of symmetrical space relations, the importance of which in regard to crystals has 
long been recognised, and whose value in relation to the fundamental concepts of 
matter generally is of late becoming more and more appreciated. The inquiry is 
a purely geometrical one, and is therefore independent of any particular concept as 
to the ultimate nature of matter. ; 
The basis of the investigation is a definition of homogeneity of structure which 
runs as follows :— 
A homogeneous structure is one every point within which, if we regard the 
! Published in full in the Mineralogical Magazine, vol. xi. p. 119; and also in 
Groth’s Zeitschrift fiir Krystallographic, vol. xxvii. p. 449. 
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