34 



In practice, however, many engineers prefer the erecting or terrestrial telescope. 

 Such telescopes must be made with an eye-piece consisting of four lenses, as by 

 adding two more lenses, objects are shown right side up, as viewed with the naked 

 eye. In the construction of an erecting eye-piece the chromatic aberration can be 

 corrected by the two additional lenses required to secure an upright image ; but in 

 the case of short and powerful telescopes the difficulties presenting themselves to 

 secure a perfectly flat field of view are very great, and recourse must often be had 

 to a compound lens. In the Kellner terrestrial eye-piece the third lens, reckoning 

 from the eye, is therefore compound, and both parts are cemented together. 



The Huyghenian eye-piece is used to a very limited extent in the more modern 

 telescopes of instruments of precision. It is most frequently met with in the 

 large telescopes used in physical astronomy, where objects are merely viewed, but 

 no measurements made. The field of view is large, but not quite flat. The amount 

 of light is greater than in the other eye-pieces. The eye-piece consists of two 

 plano-convex lenses with their convex sides facing the object-glass. The main fea- 

 tures are, that in this eye-piece the second lens is placed between the object-glass 

 and its focus, and that it brings the image to a focus at a point half-way between 

 the two lenses of the eye-piece. The focal length of the second lens is three times 

 larger than that of the eye-lens, and they are placed apart at a distance equal to 

 one-half their combined focal length. The image is viewed by the eye-lens. It is 

 called a negative eye-piece, because the image is formed at a point between the 

 lenses. 



The magnifying power of a telescope must be proportional to the aperture. If 

 the magnifying power is too high for the aperture, ordinary objects will appear too 

 faint ; and if the magnifying power is too low, the objects will appear so small 

 that the engineer cannot point upon them with sufficient accuracy. 



The magnifying power should be such that the least perceptible motion of the 

 bubble of a level, or change in the reading of the verniers, should cause sufficient 

 movement of the cross-wires over the object in the field of view to be readily notice- 

 able. A higher power than this is worse than useless, since objects are less brilliant. 

 A lower power would not develop the full capacities of the instrument. Messrs. 

 C. L. Berger & Sons adapt, therefore, the magnifying power and aperture of their 

 instruments to the sensitiveness of the levels, and the fineness of the graduation. 



In the telescopes of the instruments manufactured, by C. L. Berger & Sons, the 

 main tube has a much smaller diameter than is usual in proportion to the size of aper- 

 ture. This is accomplished without cutting off any light derived from the object- 

 glass, since the pencil of light within the telescope is continually diminishing in 

 diameter until it comes to a focus at the plane of the spider-lines. The danger of 

 an increase of reflections caused by bringing the interior surface of the telescope- 

 tube nearer to this pencil of rays, is neutralized by the introduction of several more 

 diaphragms properly placed, and by the use of a specially dead black coating for 

 the interior. By this method of construction the weight of the telescope is greatly 

 reduced compared with the large apertures used by them, and therefore there is 

 less wear on the horizontal axis of revolution, and less friction of the object-slide. 

 There is, also, on this account, less surface exposed to the wind, and the instru- 

 ment is consequently more steady. C. L. B. 



The Graduations. 



Engineers' transits have various graduations on their circles, according to the 

 requirements of the different branches of civil engineering. These various gradua- 

 tions are read by opposite verniers, which may be either single or double. Ameri- 

 can instruments have usually double opposite verniers, commonly reading the 

 circle to single minutes or to thirty seconds. For a higher grade of work, required 

 in the larger cities and on extended land surveys, they should, however, read to 

 twenty or ten seconds. 



The customary graduations of C. L. Berger & Sons' instruments are, First, the 

 circle divided to half degrees, tne verniers reading to single minutes. Second, the 

 circle divided to twenty minutes, the verniers reading to thirty seconds. Third, 

 the circle divided to fifteen minutes, the verniers reading to twenty seconds. 

 Fourth, the circle divided to twenty minutes, the verniers reading to twenty 

 seconds. Fifth, the circle divided to ten minutes, the verniers reading to ten 

 seconds. 



