33 



Description of the Telescope. 



THE object-glass is achromatic, being made of two lenses, one of crown and one 

 of flint glass. Both these lenses are made of the celebrated " Jena " glass (intro- 

 duced about 1885), which has a greater index of refraction and power of dispersion 

 than known before this time. For the most part, that is, whenever the diameter of 

 these lenses is not too large, we since 1889 cement them together so as to 

 make one lens only. In so doing the disturbing reflections from their inner sur- 

 faces, and the settling of a film between them is prevented, besides securing to the 

 telescope an additional amount of light equal to about 8 per cent. The curvatures 

 are computed from special formulas, so that the telescope may have the largest 

 aperture possible with a short focal length. 



The engineer will appreciate the slightest gain in the diameter of the object- 

 glass, since the amount of light received from any object varies as the square of 

 that diameter. Thus an object-glass 1 inches in diameter will admit half as much 

 light again as an object-glass one inch in diameter. 



The eye-piece, or ocular, as it is sometimes called, is the combination of lenses 

 used in the telescope with which the image formed at the focus of the object-glass 

 is viewed. 



The simplest and most commonly used eye-piece in the telescopes of instru- 

 ments of precision, where spider-threads and micrometers are used in making 

 measurements, is the Ramsden astronomical or positive eye-piece. It consists of 

 two plano-convex lenses, commonly of the same focus, placed apart at a distance of 

 two-thirds the focal length of either, the convex sides facing each other. It has 

 the advantage of being placed behind the focus of the object-glass. It is almost 

 free from spherical aberration, and gives a perfectly flat field of view, so that the 

 spider-threads can be seen distinctly throughout their entire length. Unfortun- 

 ately it is not entirely free from chromatic aberration, that is, not strictly achro- 

 matic, and therefore the Kellnerand Steinheil eye-pieces are frequently preferred, as 

 in them the chromatic aberration is sensibly eliminated, so that a bright object 

 viewed with a normal eye will appear achromatic, a condition as important in the 

 eye-piece as in the object-glass. 



The Kellner eye-piece, also, consists of two lenses. The one nearest the eye, or 

 eye-lens, is a compound lens composed of crown and flint-glass, as in the objective. 

 Both are cemented together so as to make one, to prevent loss of light consequent 

 upon a ray passing from one substance into another. In its common form the eye- 

 lens is plano-convex, with the plane side nearest the eye, while the second or field- 

 lens is double-convex. 



In the Steinheil eye-piece both lenses are compound, as in the eye-lens of the 

 Kellner. The parts of each lens being cemented together, they form two double- 

 convex lenses, and therefore it may be designated as an achromatic double eye-piece. 

 There are some deviations in the construction of the three eye-pieces mentioned 

 above, but mainly as to the proper curvature of the lenses and their proper distances 

 apart, depending as they do on the index of refraction and power of dispersion 

 of the glass used in the construction of the object-glass and eye-piece, but the 

 principle as above explained, by which an achromatic image is obtained, underlies 

 all of them. 



The Ramsden eye-piece is generally preferred on account of its greater simpli- 

 city and its flat field of view, which latter condition is more difficult to be obtained 

 with the Kellner and Steinheil eye-pieces in powerful telescopes of limited length, 

 on account of the somewhat larger field of view possessed by these eye-pieces. 

 Moreover, the compound lenses are liable to be affected after a while by opacities 

 caused by a crystallization, as it were, of the cement uniting the parts composing 

 them. 



Objects seen through the above-mentioned eye-pieces are, however, inverted, 

 and telescopes so constructed are often objected to on this account. It neverthe- 

 less is the most proper telescope to use where fine telescopic measurements must 

 be made, as the image is more brilliant than when the objects are shown upright, 

 and it requires but little practice to get accustomed to its use. The inverting tel- 

 escope has some other ad vantages that should be mentioned here. The eye-piece 

 being shorter, an object-glass of greater focal length is obtained in the same length 

 of telescope, thereby favoring the conditions imposed to secure the best definition 

 where the telescope must be short and powerful. Any increase in the focal length of 

 an object-glass adds to the magnifying power in the direct way, without entailing 

 the loss of light consequent upon the use of an eye-piece made unduly powerful. 

 On the other hand, an increase in the magnifying power of the eye-piece magnifies 

 the least imperfection that may exist in the object-glass, and makes the cross- 

 wires appear too coarse. 



