57 6 Proceedings of Societies . [August, 
frequently Boisbaudran’s method and wave-lengths, and having 
drawn a curve, determined the wave-lengths of the dark lines 
therefrom. It was in this way that they obtained the wave- 
lengths of the two dark lines they ascribed to potassium. It is, 
of course, just within the bounds of possibility that the line for 
which they obtained the wave-length 4045 may have been an 
iron line reversed, but, as the principal well-known lines of 
potassium were at the same time seen bright in the spectrum 
while those of iron were not seen, it is far more probable — 
almost certain — that the line was really due to potassium, and 
the last figure of the wave-length wrongly determined. To 
attempt to determine wave-lengths to any greater degree of 
nicety would, so far as their purpose was concerned, have been 
labour thrown away. Whenever the wave-length of a line 
observed by the authors to be reversed is given differently from 
that given by Thalen and that given by Boisbaudran for the 
bright line to which they assume it to correspond, it was deter- 
mined in the way above described. The authors state that by 
their method of working the very high dispersive power required 
for the specific identification of any substance by the determi- 
nation of an individual wave-length is avoided ; as they depend 
on the greatly diminished likelihood of error when several 
groups of lines of the same substance are seen to be continually 
present at the time of one or more reversals. It was the neces- 
sity of being able to rapidly sweep the entire spectrum for the 
above purpose that caused them to limit the dispersion. A large 
majority of the wave-lengths given by Thalen were obtained by 
means of the moderate dispersion of one bisulphide of carbon 
prism, a less dispersion than we have used ; and they consider 
it would be incorrect to suppose that no enduring work in this 
field of spectroscopy can be effected except with the enormous 
dispersive power which Mr. Lockyer recommends. 
“ Microscopical Researches in High Power Definition,” by 
G. W. Royston-Pigott, M.A., M.D. Cantab., F.R.S. In its 
general scope the paper is intended to deal with difficulties in 
microscopic research, usually found insuperable — such, for 
instance, as the invisibility of minute closely packed refracting 
spherules, existing in double rouleaux, or promiscuously aggre- 
gated ; when their individual diameter varies between the 
i-8o,oooth to the 1-200, oooth of an inch. These difficulties are 
principally created by overlapping images — due partly to resi- 
duary aberrations both spherical and chromatic — partly to the 
effects of diffradtion, caused by brilliant illuminations of spurious 
disks of light — partly to the constant development of Eidola or 
false images, which vary the loci of their development according 
to the nature of underlying structures ; and according to the 
object-glasses being over or under corrected — and partly, and 
indeed very considerably, created by the use of excessively large 
