60 : Scientific News. [January, 
possible from errors of all kinds, Plate No. 5 consists of 21 
lines separated by an interval of 35 mm. After careful measure- 
ment with two different niccousetate and two comparators, the 
plates were sent to Prof. Morley, the details of whose measure- 
ments will be found in the forthcoming volume of the Proceed- 
ings of the American Academy of Arts and Sciences. The 
degree of agreement between his results and my own is much 
more perfect than I had anticipated before beginning this investi- 
gation. © 
From this investigation I think we may safely draw the follow- 
ing conclusions: (a.) Two equally skillful observers can measure 
the same space within about 30v of an inch if the space does 
not exceed $y of an inch. For a space of yy of an inch the 
deviation will probably amount to gg}s9 of an inch in case the 
measurements are made with an eye piece or a filar micrometer. 
(4). The average deviation for accumulated errors under similar 
conditions is not far from gg495 of an inch for eleven intervals. 
For a larger number of intervals the deviation will be somewhat 
ae but it will not be in proportion to the number of intervals. 
(c) A single observer can obtain an agreement with a normal 
equation representing all the observed values as far as a solution 
by least squares can represent them, within somewhat smaller 
limits than those obtained by comparing the results obtained by 
two different observers— Wim. A. Rogirs, Harvard College Obser- 
vatory. (from a paper read at the National Microscopical Congress, 
August, 1878. 
Tue Society Screw.—At a recent meeting of the State Micro- 
scopical Society of Illinois, Mr. Bulloch urged the desirability of 
adopting a uniform objective screw of larger size than the society 
screw now in use, as being essential to the efficiency of low power 
lenses of high angle. That the society screw, which has now 
become an almost indispensable convenience, is too small to 
admit of efficient work from these lenses, is a conceded fact, and 
some makers in this country who make low powers of enormous 
angle have already adopted special screws for them. The uni- 
formity urged by Mr. Bulloch is greatly to be desired, and could 
be easily attained if its importance were appreciated in time. 
-ExcHAnces.—Gatherings ey alg polycystina, etc., wanted 
in exchange. e I. F. Stidham, Columbus, 
Western mosses, etc., for other SA George W. Wor- 
cester, West Side, Cead Co 
Diatomaceous earths and named dob for named diatoms or 
other good mounted objects. M. A. Booth, Longmeadow, Mass. 
:o_—_—_— 
SCIENTIFIC NEWS. 
— The United States Entomological Commission, attached to 
oe the poten States Geological and Geographical Survey of the 
