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scope with one eye, and compare the moon as seen there with the 

 same object as seen with the other and unassisted eye, and he 

 will very soon be convinced, for with a little manipulation the 

 moon, as seen by the naked eye, may be brought over the mag- 

 nified image as seen in the telescope, when it will be found that 

 the former appears so small that it might be put into one of the 

 craters of the latter. 



To perform the same experiment with my " Newtonian Reflec- 

 tor," I have constructed a small metallic speculum, which can be 

 slipped on to the eye-piece tube, and can be turned to any re- 

 quired angle, so that the object as seen by the naked eye can be 

 brought over the magnified image in the telescope. When the 

 instrument with a power of 108 is turned on the Post-office 

 clock we find that the whole diameter of the tower as seen in the 

 plane reflector occupies only about the space of one of the minutes 

 marked on the circumference of the dial as seen in the telescope. 

 By methods such as these we can convince the most sceptical that 

 telescopes do magnify, but for those who use telescopes for scien- 

 tific observation it is absolutely necessary that they should know 

 the magnifying powers of their instruments with the greatest pos- 

 sible precision. For this purpose several methods may and have 

 been adopted. As the magnifying power of any telescope depends 

 entirely on the magnitude of the angle under which any object is 

 presented to the eye in the eye-piece of the instrument, and as 

 this angle can at once be found by dividing the focal length of 

 the object glass or the speculum by that of the eye-piece em- 

 ployed, this might seem a very simple way of getting at the exact 

 power ; and indeed it is fairly satisfactory as far as low powers 

 are concerned, or with single-lens eye-pieces, but when we come to 

 use eye-pieces composed of several lenses the matter becomes more 

 difficult, and when we come to use those of a very high power the 

 exact determination of the focal length of each lens becomes a 

 matter of exceeding difficulty, and the result of such measure- 

 ments cannot be relied on. 



Another method, and one a good deal more to be depended on, 

 is to have a circular disc of white paper on a black ground at a 

 distance of 100 yards or so, and having a card with two black 

 parallel lines drawn on it whose distance is exactly equal to the 

 diameter of the paper circle, then viewing through the telescope 

 the paper circle with one eye and the parallel lines on the card 

 with the other let the lines be moved to such a distance from the 

 eye that they shall exactly correspond with the diameter of the 

 disc. The quotient found by dividing, the distance of the paper 

 disc from the eye by that of the parallel lines from the same 

 gives the magnifying power of the telescope. This method, though 

 good, requires some practice, and is only applicable to refractors 



