so that the impression which represents to us the real magnitude is the product of these two elements. When the objects, however, are at a great distance, it is more difficult to form a correct estimate of their true magnitudes. The visual angles are so small, that they prevent comparison; and the estimated bulks of the objects depend in a great measure upon the _apparent_ magnitudes; and thus an object situated at a great distance, appears to us much smaller than it is in reality. We also estimate objects to be nearer or farther distant according as they are more or less clear, and our perception of them more or less distinct and well defined; and likewise, when several objects intervene between us and the object we are particularly observing. We make a sort of addition of all the estimated distances of intermediate objects, in order to form a total distance of the remote object, which in this case appears to be farther off than if the intervening space were unoccupied. It is generally estimated that no terrestrial object can be distinctly perceived, if the visual angle it subtends be less than _one minute of a degree_; and that most objects become indistinct, when the angle they subtend at the pupil of the eye is less than six minutes. We have deemed it expedient to introduce the above remarks on the apparent magnitude of objects, because the principal use of a telescope is to increase the angle of vision, or to represent objects under a larger angle than that under which they appear to the naked eye, so as to render the view of distant objects more distinct, and to exhibit to the organ of vision those objects which would otherwise be invisible. A telescope may be said to enlarge an object just as many times as the angle under which the instrument represents it, is greater than that under which it appears to the unassisted eye. Thus the moon appears to the naked eye under an angle of about half a degree; consequently a telescope magnifies 60 times if it represents that orb under an angle of 30 degrees; and if it magnified 180 times, it would exhibit the moon under an angle of 90 degrees, which would make her appear to fill half of the visible heavens, or the space which intervenes from the horizon to the zenith. CHAPTER IV. ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES.