enses in the water, but no pressure other than the weight
of the cotton or brush should be used. The writer prefers to use the
palms of the hand with plenty of good soap on them to rub the
surfaces, although the cheese cloth and the soap answers nicely, and
there seems to be absolutely no danger of scratching when using the
hands or the cheese cloth when plenty of water is used; indeed when I
wish to wipe off the front surface of an objective in use, and the
lens cannot well be taken out, I first dust off the gross particles
and then use the cheese cloth with soap and water, and having gone
over the surface gently with one piece of cloth, throw it away and
take another, perhaps a third one, and then when the dirt is, as it
were, all lifted up from the surface, a piece of dry cheese cloth will
finish the work, leaving a clean brilliant surface, and no scratches
of any kind.

"In washing large objectives in water I generally use a 'tub' and
stand the lenses on their edge. When thoroughly washed they are taken
out and laid on a bundle of cheese cloth and several pieces of the
same used to dry them.

"I think it best not to leave them to drain dry; better take up all
moisture with the cloth, and vigorous rubbing will do no harm if the
surfaces have no abrading material on them. I have yet to injure a
glass cleaned in this way.

"This process may seem a rather long and tedious one, but it is not so
in practice, and it pays.

"In some places objectives must be frequently cleaned, not only
because they become covered with an adherent dust, but because that
dust produces so much diffused light in the field as to ruin some
kinds of telescope work. Mr. Hale of the Kenwood Observatory tells me
he cannot do any good prominence photography unless his objective has
a clean surface; indeed every observer of faint objects or delicate
planetary markings knows full well the value of a dark field free from
diffused light. The object-glass maker uses his best efforts to
produce the most perfect polish on his lenses, aside from the accuracy
of the curves, both for high light value and freedom from diffused
light in the field, and if the surfaces are allowed to become covered
with dust, his good work counts for little.

"If only the front surface needs cleaning, the method of cleaning
with cheese cloth, soap and water, as described above, answers very
well, but always throw away the first and, if necessary, the second
cloth, the