Half-hours with the Telescope eBook

Richard Anthony Proctor
This eBook from the Gutenberg Project consists of approximately 116 pages of information about Half-hours with the Telescope.
aperture had to be increased by about an inch.  I found a tubing made of alternate layers of card and calico well pasted together, to be both light and strong.  But for the full length of tube I think a core of metal is wanted.  A learned and ingenious friend, Mr. Sharp, Fellow of St. John’s College, informs me that a tube of tin, covered with layers of brown paper, well pasted and thicker near the middle of the tube, forms a light and strong telescope-tube, almost wholly free from vibration.

Suffer no inexperienced person to deal with your object-glass.  I knew a valuable glass ruined by the proceedings of a workman who had been told to attach three pieces of brass round the cell of the double lens.  What he had done remained unknown, but ever after a wretched glare of light surrounded all objects of any brilliancy.

One word about the inversion of objects by the astronomical telescope.  It is singular that any difficulty should be felt about so simple a matter, yet I have seen in the writings of more than one distinguished astronomer, wholly incorrect views as to the nature of the inversion.  One tells us that to obtain the correct presentation from a picture taken with a telescope, the view should be inverted, held up to the light, and looked at from the back of the paper.  Another tells us to invert the picture and hold it opposite a looking-glass.  Neither method is correct.  The simple correction wanted is to hold the picture upside down—­the same change which brings the top to the bottom brings the right to the left, i.e., fully corrects the inversion.

In the case, however, of a picture taken by an Herschelian reflector, the inversion not being complete, a different method must be adopted.  In fact, either of the above-named processes, incorrect for the ordinary astronomical, would be correct for the Herschelian Telescope.  The latter inverts but does not reverse right and left; therefore after inverting our picture we must interchange right and left because they have been reversed by the inversion.  This is effected either by looking at the picture from behind, or by holding it up to a mirror.

[Illustration:  PLATE II.]



Any of the half-hours here assigned to the constellation-seasons may be taken first, and the rest in seasonal or cyclic order.  The following introductory remarks are applicable to each:—­

If we stand on an open space, on any clear night, we see above us the celestial dome spangled with stars, apparently fixed in position.  But after a little time it becomes clear that these orbs are slowly shifting their position.  Those near the eastern horizon are rising, those near the western setting.  Careful and continuous observation would show that the stars are all moving in the same way, precisely, as they would if they were fixed to the concave surface of

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Half-hours with the Telescope from Project Gutenberg. Public domain.
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