A stop 1/4 inch less in diameter than the speculum, is often useful, in order to cut off the internal reflections of the tube.
With mirrors 6-1/2 inches in diameter and under stops are seldom required; with the 6-1/2 inch a 5 inch stop may however sometimes be used with advantage on very bright objects. With an 8-1/2 inch mirror a 7 inch stop often improves planetary definition.
With larger sizes than 8-1/2 inches several stops of different diameter may be used, when experiment will determine what size is best suited to the condition of the atmosphere, and the character and brilliancy of the object to be observed. These stops can be easily cut out of thin cardboard and afterwards blacked with Indian-ink or lampblack.
On Observing the Sun.
No larger aperture than 4-1/2 inches should be employed for solar observation, except with a specially constructed “Solar Eye-piece,” as even a 4-1/2 inch speculum often concentrates sufficient heat to crack the coloured glass of an ordinary sun-cap, if exposed for any length of time.
If a “solar eye-piece” is employed, the whole aperture may be used. In viewing the moon a slightly tinted glass is often most useful, especially to persons with weak sight, as it takes off a great deal of the glare.
To Silver and Polish the Specula.
The cost of silvering is trifling, and with cleanliness and ordinary care very little difficulty will be experienced. The apparatus and chemicals required consist of the following articles:—
Apparatus.—A Silvering Vessel:—This should be a flat-bottomed circular glass or glazed dish, 1 inch or more larger in diameter than the speculum to be silvered, and sufficiently deep to allow of a stratum of fluid of an inch or rather more between the face of the mirror and the bottom of the dish, the top of the mirror being nearly level with the edge of the dish.
A Mixing Vessel:—A 40 oz. glass measure will answer well, but care should be taken not to scratch the sides whilst stirring, or the glass is liable to fly. Should a measure not be procurable, any receptacle of sufficient size may be used, but a glass one is best, as it will allow of the action of ammonia being better observed.