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eye-piece ever made. Very faint objects can often be seen when looking at another part of the field, or as it is technically called, “oblique vision.” If the eye be kept a few minutes in the dark before observing faint objects, it will be more sensitive to them.

Nearly all the best drawings of planets have been made with powers of from 200 to 300.

It has also been found advantageous to occasionally shift the position of an object in the field when it is being drawn, as the eye will not be so fatigued if various portions of the retina are employed.

Several instruments for ascertaining star magnitudes have been invented, but nothing is better than an adjustable aperture.

The colour of a star can often be best estimated as an out of focus disc, especially with reflectors. Never judge the colour of a star near the horizon.

Never attempt to view the Sun without a special eye-piece. A neglect of this simple precaution may, in an instant, cause the observer the loss of his sight.

When viewing the Moon with powerful telescopes, employ a light blue or neutral tinted glass, to diminish the amount of light, which from a large aperture is perfectly unbearable, especially to observers whose eyes are weak.

Observations of Mercury and Venus are most satisfactory if made during the day; for this purpose an equatorial is almost indispensable, though both planets may be sometimes picked up without that assistance if their right ascension and declination are compared with other objects visible in the daylight. Proctor has well shown how this may be done in one of his most useful “ Half Hours.”

Of the satellites of Saturn, a telescope of 2-inches aperture should show Titan; one of 3-inches, Titan and Iapetus ; 4-inches, both these, Rhea, and Dione ; 6-inches, all the former, Tethys, and perhaps Enceladus; the fainter satellites cannot possibly be seen with a less aperture than 1 o-inches, and that with exceptionally favourable definition.

Stops can be cut out of cardboard, or thin zinc, or tin; and should always be blacked with some lamp-black and gold size.

Stops should be supported a few inches in front of the tube of a reflector, in order that the air may have free egress between the end of the tube and the stop, and not pass through the stop.

Stops are seldom required with telescopes of less than 4 inches aperture, unless the edge of the object glass or mirror is imperfect. As a rule specula are worked in discs of the required size, and therefore the extreme edge is often slightly defective. This is the reason why there are 6J, 8^-, 10J and 12J mirrors, it being supposed that the marginal |-inch shall be covered with a stop. But many mirrors perform admirably up to the extreme edge, and only require stops from defective atmospheric conditions. With large telescopes a series of stops of various sizes are most useful.

The first stop should be just enough to cover the extreme margin,

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Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 99,32 %.

La langue de reconnaissance de l'OCR est l'Anglais.

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