e arguments are unreliable and incorrect.

"It is a well-known fact that when a faint object has been once seen
through a telescope, others are able to see it through a smaller
instrument. This was the case with the satellites of Mars, which have
been seen with much smaller instruments than that used to discover them.

"The fact that such objects are really seen is proved by the observer
marking them on his drawing in their correct position, although they may
have moved from the point at which they were originally seen.

"I will give you an illustration of the ease with which it is possible
to overlook something that should be clearly visible to you, yet it is
not seen by you until your attention is called to it by some one else.
Almost every one has had some such experience:--

"You may have on the front of your coat a small stain, or grease-spot,
in a position where you could plainly see it, yet might wear the coat
for days or even weeks in complete unconsciousness of the existence of
the stain until some one pointed it out to you. After that you cannot
look at the coat without seeing the stain, and it becomes so
persistently obtrusive that you are compelled to have it removed. There
is, however, no imagination about your seeing the mark."

John here said to me: "Professor, I noticed you said that many who do
not believe in the actuality of the lines and markings on Mars
frequently refer to the fact that, while they are stated to be seen
through small telescopes, they are quite invisible through a very large
instrument, and they regard this as proving that the lines or markings
do not exist. Is there not something in this argument?"

"Well, John," I replied, "the argument sounds not only plausible, but
reasonable, and inexperienced persons might use the argument, believing
it to be a sound and good one. I must, however, confess that I have
been surprised to see this argument used by persons who must surely know
that there is no weight in it at all.

"It is well known to all practical observers, and indeed to all who have
studied optical matters, that, for several reasons, very large
telescopes are quite unsuited for the observation of fine planetary
detail.

"The real advantage of these enormous instruments lies in their great
'light-grasp,' which enables observers to see very faint points of
light, such as small satellites of planets, faint stars, double stars,
distant comets, or nebulae, which could not be s