bservations of an object near Mars, and asked me what I thought
of them. I remarked, "Why, that looks very much like a satellite."

Yet he seemed very incredulous on the subject; so incredulous
that I feared he might make no further attempt to see the object.
I afterward learned, however, that this was entirely a misapprehension
on my part. He had been making a careful search for some time,
and had no intention of abandoning it until the matter was cleared
up one way or the other.

The possibility of the object being an asteroid suggested itself.
I volunteered to test this question by looking at the ephemerides
of all the small planets in the neighborhood of Mars. A very little
searching disproved the possibility of the object belonging to this
class. One such object was in the neighborhood, but its motion was
incompatible with the measures.

Then I remarked that, if the object were really a satellite, the
measures already made upon it, and the approximately known mass of
the planet, would enable the motion of the satellite to be determined
for a day or two. Thus I found that on that night the satellite would
be hidden in the early evening by the planet, but would emerge after
midnight. I therefore suggested to Professor Hall that, if it was
not seen in the early evening, he should wait until after midnight.
The result was in accordance with the prediction,--the satellite
was not visible in the early evening, but came out after midnight.
No further doubt was possible, and the discovery was published.
The labor of searching and observing was so exhausting that Professor
Hall let me compute the preliminary orbit of the satellites from
his early observations.

My calculations and suggestions lost an importance they might
otherwise have claimed, for the reason that several clear nights
followed. Had cloudy weather intervened, a knowledge of when to
look for the object might have greatly facilitated its recognition.

It is still an open question, perhaps, whether a great refracting
telescope will last unimpaired for an indefinite length of time.
I am not aware that the twin instruments of Harvard and Pulkowa,
mounted in 1843, have suffered from age, nor am I aware that any
of Alvan Clark's instruments are less perfect to-day than when they
left the hands of their makers. But not long after the discovery of
the satellites of Mars, doubts began to spread in some quarters as to
whether the great Washington telesco