220 THE POPULAR SCIENCE MONTHLY. 



the magnifying power which an eyepiece gives when applied to a 

 telescope it is necessary to know the equivalent, or combined, 

 focal length of the two lenses. Two simple rules, easily remem- 

 bered, supply the means of ascertaining this. The equivalent 

 focal length of a negative or Huygens eyepiece is equal to half 

 the focal length of the larger or field lens. The equivalent focal 

 length of a positive or Ramsden eyepiece is equal to three 

 fourths of the focal length of either of the lenses. Having ascer- 

 tained the equivalent focal length of the eyepiece, it is only neces- 

 sary to divide it into the focal length of the object glass (or mir- 

 ror) in order to know the magnifying power of your telescope 

 when that particular eyepiece is in use. 



A first-class object glass (or mirror) will bear a magnifying 

 power of one hundred to the inch of aperture when the air is in 

 good condition that is, if you are looking at stars. If you are 

 viewing the moon, or a planet, better results will always be ob- 

 tained with lower powers say fifty to the inch at the most. And 

 under ordinary atmospheric conditions a power of from fifty 

 to seventy-five to the inch is far better for stars than a higher 

 power. With a five-inch telescope that would mean from two 

 hundred and fifty to three hundred and seventy-five diameters, 

 and such powers should only be applied for the sake of separating 

 very close double stars. As a general rule, the lowest power that 

 will distinctly show what you desire to see gives the best results. 

 The experienced observer never uses as high powers as the begin- 

 ner does. The number of eyepieces purchased with a telescope 

 should never be less than three a very low power say ten to the 

 inch ; a very high power, seventy-five or one hundred to the inch, 

 for occasional use ; and a medium power say forty to the inch 

 for general use. If you can afford it, get a full battery of eye- 

 pieces six or eight, or a dozen for experience shows that differ- 

 ent objects require different powers in order to be best seen, and, 

 moreover, a slight change of power is frequently a great relief to 

 the eye. 



There is one other thing of great importance to be considered 

 in purchasing a telescope the mounting. If your glass is not 

 well mounted on a steady and easily managed stand, you might 

 better have spent your money for something more useful. I have 

 endured hours of torment while trying to see stars through a tele- 

 scope that was shivering in the wind and dancing to every mo- 

 tion of the bystanders, to say nothing of the wriggling contor- 

 tions caused by the application of my own fingers to the focussing 

 screw. The best of all stands is a solid iron pillar firmly fastened 

 into a brick or stone pier, sunk at least four feet in the ground, 

 and surmounted by a well-made equatorial bearing whose polar 

 axis has been carefully placed in the meridian. It can be readily 



