Yet in such a case the telescope is not in fault.  The planet really appears of the estimated size.  In fact, it is often possible to prove this in a very simple manner.  If the observer wait until the planet and the moon are pretty near together, he will find that it is possible to view the planet with one eye through the telescope and the moon with the unaided eye, in such a manner that the two discs may coincide, and thus their relative apparent dimensions be at once recognised.  Nor should the indistinctness and incompleteness of the view be attributed to imperfection of the telescope; they are partly due to the nature of the observation and the low power employed, and partly to the inexperience of the beginner.

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,
      AEtheraque 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.

[Illustration:  Fig. 1.]