PLEASURES OF THE TELESCOPE. 



219 



properties of object glasses and mirrors, but a word should be 

 added concerning eyepieces. Without a good eyepiece the best 

 telescope will not perform well. The simplest of all eyepieces is 

 a single double-convex lens. With such a lens the magnifying 

 power of the telescope is measured by the ratio of the focal length 

 of the objective to that of the eye lens. Suppose the first is sixty 

 inches and the latter half an inch ; then the magnifying power 

 will be a hundred and twenty diameters i. e., the disk of a planet, 

 for instance, will be enlarged a hundred and twenty times along 

 each diameter, and its area will be enlarged the square of a hun- 

 dred and twenty, or fourteen thousand four hundred times. But 

 in reckoning magnifying power, diameter, not area, is always con- 

 sidered. For practical use an eyepiece composed of an ordinary 

 single lens is seldom advantageous, because good definition can 

 only be obtained in the center of the field. Lenses made accord- 

 ing to special formulae, however, and called solid eyepieces, give 

 excellent results, and for high powers are often to be preferred to 

 any other. The eyepieces usually furnished with telescopes are, 

 in their essential principles, compound microscopes, and they are 

 of two descriptions, " positive " and " negative/' The former gen- 

 erally goes under the name of its inventor, Ramsden, and the lat- 

 ter is named after the great Dutch astronomer, Huygens. The 

 Huygens eyepiece consists of two plano-convex lenses whose focal 



FIG. 4. NEGATIVE EYEPIECE. 



FIG. 5. POSITIVE EYEPIECE. 



lengths are in the ratio of three to one. The smaller lens is placed 

 next to the eye. Both lenses have their convex surfaces toward 

 the object glass, and their distance apart is equal to half the sum 

 of their focal lengths. In this kind of eyepiece the image is 

 formed between the two lenses, and if the work is properly done 

 such an eyepiece is achromatic. It is therefore generally pre- 

 ferred for mere seeing purposes. In the Ramsden eyepiece two 

 plano-convex lenses are also used, but they are of equal focal 

 length, are placed at a distance apart equal to two thirds of the 

 focal length of either, and have their convex sides facing one an- 

 other. With such an eyepiece the image viewed is beyond the 

 farther or field lens instead of between the two lenses, and as this 

 fact renders it easier to adjust wires or lines for measuring pur- 

 poses in the focus of the eyepiece, the Ramsden construction is 

 used when a micrometer is to be employed. In order to ascertain 



