It is to such a beginner that the following pages are specially addressed, with the hope of affording him aid and encouragement in the use of one of the most enchanting of scientific instruments,--an instrument that has created for astronomers a new sense, so to speak, by which, in the words of the ancient poet:
Subjecere oculis distantia sidera nostris, Ætheraque ingenio supposuere suo.
In the first place, it is necessary that the beginner should rightly know what is the nature of the instrument he is to use. And this is the more necessary because, while it is perfectly easy to obtain such knowledge without any profound acquaintance with the science of optics, yet in many popular works on this subject the really important points are omitted, and even in scientific works such points are too often left to be gathered from a formula. When the observer has learnt what it is that his instrument is actually to do for him, he will know how to estimate its performance, and how to vary the application of its powers--whether illuminating or magnifying--according to the nature of the object to be observed.
Let us consider what it is that limits the range of _natural_ vision applied to distant objects. What causes an object to become invisible as its distance increases? Two things are necessary that an object should be visible. It must be _large_ enough to be appreciated by the eye, and it must _send light_ enough. Thus increase of distance may render an object invisible, either through diminution of its apparent size, or through diminution in the quantity of light it sends to the eye, or through both these causes combined. A telescope, therefore, or (as its name implies) an instrument to render distant objects visible, must be both a magnifying and an illuminating instrument.
Let EF, fig. 1, be an object, not near to AB as in the figure, but so far off that the bounding lines from A and B would meet at the point corresponding to the point P. Then if a large convex glass AB (called an _object-glass_) be interposed between the object and the eye, all those rays which, proceeding from P, fall on AB, will be caused to converge nearly to a point _p_. The same is true for every point of the object EMF, and thus a small image, _emf_, will be formed. This image will not lie exactly on a flat surface, but will be curved about the point midway between A and B as a centre. Now if the lens AB is removed, and an eye is placed at _m_ to view the distant object EMF, those rays only from each point of the object which fall on the pupil of the eye (whose diameter is about equal to _mp_ suppose) will serve to render the object visible. On the other hand, every point of the image _emf_ has received the whole of the light gathered up by the large glass AB. If then we can only make this light _available_, it is clear that we shall have acquired a large increase of _light_ from the distant object. Now it will be noticed that the light which has converged to _p_, diverges from _p_ so that an eye, placed that this diverging pencil of rays may fall upon it, would be too small to receive the whole of the pencil. Or, if it did receive the whole of this pencil, it clearly could not receive the whole of the pencils proceeding from other parts of the image _emf_. _Something_ would be gained, though, even in this case, since it is clear that an eye thus placed at a distance of ten inches from _emf_ (which is about the average distance of distinct vision) would not only receive much more light from the image _emf_, than it would from the object EMF, but see the image much larger than the object. It is in this way that a simple object-glass forms a telescope, a circumstance we shall presently have to notice more at length. But we want to gain the full benefit of the light which has been gathered up for us by our object-glass. We therefore interpose a small convex glass _ab_ (called an eye-glass) between the image and the eye, at such a distance from the image that the divergent pencil of rays is converted into a pencil of parallel or nearly parallel rays. Call this an emergent pencil. Then all the emergent pencils now converge to a point on the axial line _m_M (produced beyond _m_), and an eye suitably placed can take in all of them at once. Thus the whole, or a large part, of the image is seen at once. But the image is seen inverted as shown. This is the Telescope, as it was first discovered, and such an arrangement would now be called a _simple astronomical Telescope_.