207 
NOTES ON OPTICAL INSTRUMENTS. 
BY 
CAPTAIN D. G. PRINSEP, R.A. 
Considering that optical instruments are now largely used in the ser¬ 
vice, a few remarks on their care and preservation are here jotted down 
as they have occurred to the writer. To enter into the most elementary 
principles of light would take more room than could be spared, and, 
therefore, for any elucidation of principles I must refer my readers to 
“ Mathematical Instruments,” by Heather (published by Crosby, Lock- 
wood and Son), or to any other treatise on light. I include a few 
practical rules which are in use with Messrs. Cary, Opticians to the 
Navy, 181, Strand, W.C., who have afforded me the greatest assistance, 
and will be pleased to give any further information to those requiring it. 
Optical instruments should always be kept in a dry place at about 
the same temperature as the air outside, and should never be put 
away wet or damp in their cases, but brought in and cleaned as soon 
as possible and then replaced. 
In taking a telescope to pieces to clean the lenses be very careful to 
replace each combination of lenses in the same order as when first taken 
out. It is best and safest to only clean one set at a time, leaving the 
others in their places until the first set is completely replaced. 
A soft cameTs hair brush is best for removing particles of dust from 
the glasses. They may then be rubbed very carefully and gently with 
an old fine cambric handkerchief {silk will scratch lenses in time). Any¬ 
thing employed for cleaning lenses should be kept in a box when not in 
use. A drop of spirits of wine is efficient for removing refractory 
stains. 
If the lenses are discoloured from the action of the atmosphere, 
try washing them in potash dissolved in hot water, but usually re¬ 
polishing is necessary. 
If caused by the cement becoming discoloured from age; warm the 
glasses to separate them, then clean with a rag dipped in alcohol. Put 
a little Canada balsam on the surfaces to be united, work out the 
bubbles of air (if any) and warm, then allow to cool, and clean the 
edges with alcohol. All lenses have a small mark, either a scratch or 
pencil mark, on the circumference which should coincide with a similar 
mark on the lens to be united to it, and care must be taken that these 
marks do coincide or distortion will ensue. 
Some lenses are not cemented together at all, but this varies with 
makers. The surfaces of the lenses are ground to different curves 
which fit together exactly, and the same surfaces must always be placed 
together. This can be ascertained by gently pressing one glass on 
4, vol. x?;. 
