18 
MESSES. DE LA RUE, STEWABT, AND LOEWY’S RESEAECHES 
parts, in directions which are perpendicular to one another. On Plate I., which accom- 
panies this paper, all measurements of those lines which have the same direction are 
designated W W', i. e. along a radius W W', while those in a direction perpendicular to 
the former are designated “ along the radius It R'.” 
All measurements were made with the greatest possible care by means of the Kathe- 
tometer of the Observatory, which reads to x q Q 0 of an inch. 
17. Every line in the preceding figure being thus measured on the average about five 
times, so as to reduce errors as far as possible, the means of the measurements were 
taken ; and of those lines for which a sufficient number of measurements existed, so as 
to trace their behaviour across the field with some degree of certainty, these measure- 
ments were represented in the form shown in Plate I. (A), where, however, only six lines 
are thus shown, although in reality for the behaviour in the direction W W' ten distinct 
lines, and for that in the direction R R/ eight lines gave extensive data for our purpose, 
and therefore eighteen lines were finally employed for our mean results. 
The mean radius of the disk produced by the whole light falling upon the plates was 
found to be 8*267 inches; and this length was subdivided into seven spaces, giving 0*267 
inch to the central portion, and drawing concentric circles at distances of half an inch 
from each other; the central portion, having a radius of 0*267 inch, was considered as 
the centre ; and, from the means of the measurements as above indicated, the behaviour 
of every line was determined at the centre and at each intersection of these concentric 
circles with the two radii W W', R R'. These means are given in the following two 
Tables, which contain also the behaviour of each separate line, which we have called 
1, 2, 3, &c., each of these numbers representing in fact a different object. 
Behaviour of eighteen lines, photographed at different distances from the centre of the 
field in the Kew Photoheliograph. 
A. Direction WW'. 
Distance 
from centre, 
in inches. 
Absolute length of each line in English inches at the corresponding distance. 
1 . 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
j Means. 
+ 3*267 
•1500 
•1533 
•1730 
•1655 
•1987 
•2187 
•2017 
*1963 
•2103 
•2103 
•1878 
+ 3-000 
*1494 
•1526 
•1708 
•1647 
•1984 
•2156 
•2009 
•1964 
•2100 
•2100 
•1869 
+ 2*500 
•1482 
•1515 
•1666 
•1634 
•1976 
•2094 
•1992 
*1967 
•2084 
•2084 
•1849 
+ 2-000 
•1469 
•1508 
•1627 
•1623 
•1969 
•2027 
•1985 
*1971 
•2071 
•2071 
•1831 
+ 1-500 
•1457 
•1489 
•1587 
•1610 
•1962 
•1962 
•1955 
*1974 
•2060 
•2060 
•1812 
+ 1-000 
•1446 
•1479 
•1547 
•1598 
•1955 
•1896 
•1938 
•1978 
•2046 
•2046 
•1793 
+ 0-500 
•1408 
•1439 
•1556 
•1586 
•1950 
•1852 
•1920 
•1980 
•2035 
•2035 
*1776 
Centre. 
•1370 
•1400 
•1586 
•1575 
•1952 
•1859 
•1955 
•1980 
•2023 
•2017 
•17 72 
-0-500 
•1435 
•1424 
•1539 
•1621 
•1952 
•1900 
•1994 
•1980 
•2008 
*1989 
•1784 
-1-000 
•1446 
•1447 
•1529 
•1623 
•1943 
•1894 
*1936 
•1900 
•2050 
•2058 
•1783 
-1-500 
•1456 
•1472 
•1564 
•1604 
*1970 
•1928 
•1959 
•1950 
•2074 
•2138 
•1812 
-2-000 
•1467 
•1495 
•1598 
•1594 
•2000 
•1942 
•1980 
•2010 
•2097 
•2139 
•1832 
— 2-500 
•1509 
•1526 
•1630 
•1595 
•2030 
*1967 
•2000 
•2070 
•2120 
•2144 
•1859 
-3-000 
•1581 
•1566 
•1644 
•1664 
•2060 
•1998 
•2020 
•2128 
•2145 
•2145 
•1895 
-3-267 
•1615 
•1584 
•1647 
•1723 
•2078 
•2015 
•2033 
•2160 
•2147 
•2156 
•1916 ! 
