to the cell, and to one another, and
if they art not marked they should be marked on the edges
conspicuously with a hard lead pencil, so that when separated they may
be put together in the same way, and placed in the same relation to
the cell. With only ordinary precaution this should be an easy
matter.

"Setting the objective on edge the two lenses may be readily
separated.

"And now as to the cleaning of the lenses. I have, on rare occasions,
found the inner surfaces of an object glass covered with a curious
film, not caused directly by moisture but by the apparent oxidation of
the tin-foil used to keep the lenses apart. "A year or more ago a
7-inch objective made by Mr. Clark was brought to me to clean. It had
evidently been sadly neglected. The inside of the lenses were covered
with such a film as I have mentioned, and I feared the glass was
ruined. When taken apart it was found that the tin-foil had oxidised
totally and had distributed itself all over the inner surfaces. I
feared the result, but was delighted to find that nitric acid and a
tuft of absorbent cotton cut all the deposit off, leaving no stains
after having passed through a subsequent washing with soap and water.

"I mention this as others may have a similar case to deal with.

"For the ordinary cleaning of an objective let a suitable sized
vessel, always a wooden one, be thoroughly cleaned with soap and
water, then half filled with clean water about the same temperature as
the glass. Slight differences of temperature are of no moment. Great
differences are dangerous in large objectives.

"I usually put a teaspoonful of ammonia in half a pail of water, and
it is well to let a piece of washed 'cheese cloth' lie in the pail, as
then there is no danger if the lens slips away from the hand, and, by
the way, every observatory, indeed every amateur owning a telescope,
should have plenty of 'cheese cloth' handy. It is cheap (about 3 cts.
per yard) and is superior for wiping purposes to any 'old soft silk
handkerchief,' chamois skin, etc. Before using it have it thoroughly
washed with soap and water, then rinsed in clean water, dried and laid
away in a box or other place where it can be kept clean. When you use
a piece to clean an objective throw it away, it is so cheap you can
afford to do so.

"If the lenses are very dirty or 'dusty,' a tuft of cotton or a
camel's-hair brush may be used to brush off the loose material before
placing the l