27 



adjustment of the telescope in the vertical plane, an instrument should be reversed and 

 an angle should be repeated. These remarks apply equally to transits made with the 

 telescopes in an eccentric position. If the line of collimation is truly at right angles to 

 the horizontal axis of revolution, the amount of the offset from the line through the 

 center of the instrument to the line of collimation will equal the eccentricity of the latter, 

 and will remain the same whether the sights be long or short. As a rule, however, the 

 small geodetic instruments of the latter class cannot be constructed with the same de- 

 gree of perfection as those with the telescope in the center : and in consequence the 

 engineer using such instruments will have to rely upon methods of observing that will 

 eliminate all instrumental errors. 



In the engineer's wye level the line of collimation must be truly concentric with 

 the object-slide and outer rings ; and it is also necessary that the telescope be well bal- 

 anced from the center of the instrument, in order to project a truly horizontal line. 



Difficult of attainment as the foregoing conditions may seem, it is proper to say 

 that improved tools, and a generally better understanding of the principles governing 

 a telescope and its relation to the instrument, have done so much toward the perfec- 

 tion of geodetic instruments, that while it may not always be possible to make an 

 instrument in which the line of sight for both wires remains true for all distances, that 

 result can generally be secured, for at least the principal wire, without requiring any 

 other but the regular cross- wire adjustment 



By the foregoing explanation it will be readily understood that it is of great im- 

 portance to have the focussing slide of such a telescope truly fitted, in order that the 

 optical axis of the object-glass may coincide with the geometrical axis of the telescope, 

 whether this slide moves in the main tube and carries the object-glass, as is the custom 

 now in the smaller instruments ; or whether it moves in special rings provided for it in 

 the main tube at the eye-end, where it will contain the eye-piece and the cross-wires, 

 as is the case in all larger instruments. Any lateral motion in the focussing slide that 

 carries the object-glass or the cross-wires, will, therefore, derange the adjustment of the 

 line of collimation. However, it is equally as clear that a wabbling of a focussing slide 

 carrying an eye-piece which serves only the purpose of a compound microscope for 

 close observations of the wires and the image of an object, is of no account save that 

 such lateral motion may be so great that the obliquity which the optical axis of the 

 eye-piece may at times have with respect to the optical axis of the telescope, may cause 

 some parallax, if the wire and image under observation are not sharply focussed to- 

 gether. In concluding, it may not be considered amiss for a full understanding of 

 this subject, to also mention in this connection, that any transparent substance, such 

 as prisms* lenses, or s/iacfe-^asses,introduced between the object sighted at and the 

 object-glass, will deflect the line of sight from its true course, unless such parts can 

 be made optically and mechanically perfect, which is rarely the case without elaborate 

 adjusting apparatus. The introduction of a lens or lenses between the object-glass 

 and wires, or that of a glass micrometer, will also have the tendency to deflect the 

 optical axis and affect the line of collimation. For this reason " Porro's telescope," 

 which requires a lens between the object-glass and the wires, complicates the above 

 conditions of a measuring telescope ; and while it may prove of some value in stadia 

 measurements, can never be adapted for the engineer's transit so long as the proper 

 functions of the transit telescope, as explained above, are considered of the greatest 

 importance. The successful performance of an instrument should not be sacrificed 

 for the sake of some doubtful novelty. 



The proper way of attaching prisms and colored glasses necessary to make sun 

 and star observations is to put them upon the eye-piece of a telescope. After the rays 

 from an object have passed through the object-glass and the plane containing the 

 wires, the line of sight as fixed by the object, optical axis, and the wires, cannot be 

 changed by additional refraction. The best way, therefore, is to apply prisms and 

 shade-glasses between the eye and the lens nearest the eye. 



Aluminum for Instruments of Precision. 



In consequence of recent improvements in the production of pure aluminum and a 

 corresponding great reduction in its cost, we frequently receive inquiries as to the adapt- 

 ability of this metal for the manufacture of engineers' and surveyors' field instruments. 



We may be permitted to say, that while we were among the earliest advocates 

 of aluminum and its alloys for mathematical instruments (see Scientific American, 

 Feb. 1, 1868), we are not so sanguine concerning its adoption for the finest class of 



* The object prism, so called, attachable to the object end of a mining telescope to aid in steep sighting, 

 from its position between the object glass and the object sighted at, must of necessity be of very limited useful- 

 ness, since the slightest change of the prism or its mounting or a change of the position of the telescope itself or 

 of its object slide will almost certainly deflect the line of sight from its true course and give no satisfactory results. 



