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TRANSACTIONS OF SCIENTIFIC SECTION. 87 
significant figures in these values. Moreover, the same 
eye is not equally sensitive to all colors. Nevertheless, 
it is possible to arrive at a fair average ‘‘ limit of visibil- 
ity’ for mass, which shall be as nearly correct as is the 
‘‘limit of visibility’ for extension, and which can be 
accepted as a fact of the same order of certainty as the 
fact embodied in the statement that ten inches is the 
*“limit of distinct vision.”’ 
Mass Limits of Visibility. 
- Gram. 
Hovassium: Chromates. oss. oo 8, 0.0000000000248 
otassimy Dichromate'.:. 22. ..60.0. 830 0.0000000000287 
Potassium Permanganate.............. 0.0000000000081 
Potassium Ferricyanide............... 0.0000000000264 
Potassium Ferrocyanide.............. 0.0000000060744 
COMMER OULPNALE I GLa os le. 0.0000000036450 
Malachite Green... . oie ote veel ee. 0.00000000000049 
APRIL 24, 1888. SIXTY-FOURTH REGULAR MEETING. 
Charles N. Arnold, chairman presiding; eight 
members and eleven guests present. 
Charles B. Warring, Ph.D., read the following 
paper : 
REAL OR SUPPOSED CHANGES OF LATITUDE. 
In Professor C, A. Young’s address before A. A. A. 
S., at Philadelphia in 1884, he says, ‘‘The observations 
by Nyren at Pulkowa seem to show aslow, steady dim- 
inution of the latitude of that observatory during the 
last 25 years, of about 1” in a century, as if the north 
' pole was drifting away, and increasing its distance from 
Pulkowa about one foot ina year. The Greenwich and 
Paris observatories do not show any such result; but 
they are not conclusive on account of the difference of 
longitude, to say nothing of their inferior precision. 
37 
