ES ts eee PN Ee NSS ER, 
to the Determination of Astronomical data. 29 
r. De la Rue speaks of having made some observations to 
iL avieine whether in the process of drying the images had un- 
dergone any distortion, sa says: ‘‘ The result, however, proved 
that there was no appreciable onteasthat. except in thickness, 
and that the soot jon film did not become di storted, rovided 
Rue in the solar eclipse of 1860, and their zero being seit 
mined in the same manner. This memoir contains a large 
amount of interesting information concerning the position and 
areas of the solar aes but the determinations of position are 
vitiated in some degree by the optical distortion of the instru- 
ment. The observers at Rew have made experiments to deter- 
mine the amount of distortion, but no definitive result has yet 
been reached. They say that the following facts may be re- 
peed: as established: “1st, that the image of any object pe 
tographically depicted is liable to a distortion, which varies at 
‘erent distances from the center of the field, and the amount 
of which may be determined for every instrument by methods 
similar to that employed by ourselves ; 2d, that in our case the 
ae of an object is larger when formed near the edge of the 
than at the center, and that the amount of elongation of a 
unit of length at the center increases with its distance from the 
center.” Their conclusion is that the ing is not sufficiently 
far advanced to justify any corrections of t itions of the 
Spots on account of the effect of distortion, but they express the 
al that at last they will be able to give thoroughly scone 
ga gaan for the effect of displacement in instrumen 
oe a ont of ae = eclipse of August, 1869, bare 
not yes so far as I know, but it is 
| pes | pee Put ee i teey i are affected by distortion 
will in a measure render the results dependent and un- 
trustworthy. It is to be hoped, however, that these pho- 
