S. P. Langley— Transit Observation. 55 
yellow precipitate falls, which continues to increase for some 
time, and which may easily be washed by decantation. The 
method of employing it is by dissolving it in an aqueous solution 
of neutral potassic oxalate. The latter substance is to be dis- 
solved in about three times its weight of hot water, and the pre- 
cipitated and washed ferrous oxalate is dissolved in it to satura- 
tion, A deep red solution is obtained, which for use needs only 
to be diluted with from five or six up to twenty or thirty times 
its bulk of water, according to the energy of effect desired. 
_An active solution of ferrous oxalate may be obtained by 
simply adding a solution of ferrous sulphate to one of neutral 
potassic oxalate, in quantity just sufficient to avoid forming a 
permanent precipitate. But the first described method gives 
the best results. 
Philadelphia, May 8, 1877. 
Art. VIIL—On the possibility of Transit Observation without 
Personal Error ; by 8. P. LANGLEY. 
swe should travel at the same apparent rate as the star. 
forming a part of the telescope. These difficulties have never 
been overcome,—scarcely attacked, and were they vanquished, 
others would remain. This method then, though perhaps still 
Susceptible of success with skillful treatment, has not yet prove 
of any practical utility. A very different proposition, originally 
due I think to M. Faye, and made as long since as 1858, is 
that for the use of photography in transits. This evidently, if 
peneable, does away with personal error, and if the exposure 
reduced to a very brief time—let us say to 0°-01—it is clear 
that the motion of ‘the star is immaterial, for we have its post- 
on photographed relatively to the wires, as it was at a given 
‘nstant. This method has been little used, except for the sun, 
owing to defective light, but though not yet practicable save 
in limited cases, it is perhaps not too much to say that it will 
Probably be the method of the future. 
