ON SOLAE PHYSICS. 
19 
B. Direction R R'. 
Distance 
from centre, 
in inches. 
Absolute length of each line in English inches at the corresponding distance. 
11. 
12. 
13. 
14. 
| 15. 
| ,, 
17. 
18. 
Means. 
+ 2-500 
•1800 
•2008 
•2216 
•2410 
•2630 
•2890 
•3020 
•3250 
•2528 
+ 2-000 
•1735 
•1823 
•2070 
•2285 
•2480 
•2749 
•2925 
•3180 
•2406 
+ 1-500 
•1651 
•1838 
•2026 
•2264 
•2440 
•2688 
•2868 
•3104 
•2348 
+ 1-000 
•1576 
•1850 
•2052 
•2303 
•2466 
•2684 
•2824 
•3025 
•2348 
+ 0-500 
•1660 
•1866 
•2078 
•2249 
•2465 
•2633 
•2774 
•2990 
•2339 
Centre 
•1672 
•1812 
•1998 
•2194 
•2395 
•2584 
•2767 
•2976 
•2300 
- 0-500 
•1702 
•1910 
•2071 
•2314 
•2499 
•2681 
•2882 
•3050 
•2389 
-1-000 
•1695 
•1922 
•2114 
•2333 
•2520 
•2729 
•2928 
•3112 
•2419 
-1-500 
•1708 
•1924 
•2135 
•2329 
•2532 
•2748 
•2950 
•3150 
•2434 
-2-000 
•1760 
•1946 
•2142 
•2351 
•2595 1 
•2771 
•2970 
•3177 
•2464 
-2-500 
•1812 
•1970 
•2149 
•2371 
•2653 
•2794 
•2994 
•3202 
•2493 
18. The mean results are graphically represented on Plate I. (B) in three curves, 
which have the distance from the centre for their abscissae, while the mean length of the 
lines are the ordinates ; both coordinates are given in inches. The curve W W' shows 
the behaviour of all lines measured in one direction, the curve EK' that of those 
measured in the direction at right angles to the former, and, finally, M M' exhibits the 
mean of both. The irregularity of the curve EE' is partly owing to the circum- 
stance that the number of observations on which it is founded is considerably less 
than those at our disposal for the curve W W ' ; and it is our intention to repeat the 
whole investigation under more favourable circumstances. It is particularly our wish 
to place the determination on a more correct basis by erecting at a considerable distance 
from the photoheliograph a suitable scale of equal parts, which when repeatedly photo- 
graphed in different positions, will in our opinion supply us with very correct constants 
of distortion. A similar contrivance would be useful in ascertaining the optical distor- 
tion of ordinary photographic lenses. In the meantime the following two facts may 
be regarded as established: — 1st, that the image of any object photographically de- 
picted is liable to a distortion, which varies at different distances from the centre of the 
field, and the amount of which may be determined for every instrument by methods 
similar to that employed by ourselves ; 2nd, that in our case the image of an object is 
larger when formed near the edge of the field than at the centre, and that the amount 
of elongation of a unit of length at the centre increases with its distance from the centre. 
Taking the most trustworthy of our curves, W W', as a preliminary result, we find the 
following values for the unit at the centre, when it passes to other positions in the field, 
the latter being given in fractions of the radius : — 
Distance in parts of radius. 
+ 1-000 
+ 0918 
+0-765 
+0-612 
+ 0-459 
+0-306 
Corresponding length of central unit. 
1+6082 
1-05474 
1-04345 
1*03330 
1-02257 
1*01185 
