DESCRIPTION 



OP TUB 



Essential Features of Our Instruments. 



Graduation. 



This very important part of a good instrument we guarantee exact and accurately 

 centered, opposite verniers reading the same. The lines are straight, thoroughly 

 black and uniform in width. There are two double verniers in every transit to read 

 angles with great rapidity as well as to make four separate readings at every sight, 

 when extreme accuracy in the repetition of angles is required. The horizontal 

 circle is graduated from to 360 with two sets of figures, running in opposite 

 directions ( unless ordered differently,) and the verniers are marked A and B . The 

 figures are large and distinct, and to avoid mistakes in reading, the figures of these 

 two sets of graduations, and those on the verniers, are inclined in opposite directions, 

 thus indicating the directions in which the verniers should be read. 



Instruments intended for mining and mountain, use can have the verniers so 

 placed that they may be read without changing the position of the engineer after 

 sighting through the telescope. 



Glass covers protect the arc and verniers from exposure. For ease in reading 

 the verniers, we have added to most of our instruments two plates of ground glass, 

 which cast a very clear light on the verniers, in any position. We recommend this 

 addition to all of our more complete transits. 



The graduations on our transits are either on brass and silvered, or else gradu- 

 ated on solid silver. The former we can only recommend for the more ordinary 

 instruments, since imperfections in the brass or composition castings frequently 

 impair the graduations, and the silvering is apt to tarnish with time and exposure. 



To graduate on solid silver adds $10 to the first outlay for the instrument, but its 

 many advantages, great permanency and smoothness of surface render it the only 

 satisfactory surface for fine graduations. 



The Telescope. 



All of its lenses are ground especially for us, by the best opticians. The teles- 

 cope is perfectly achromatic, and designed to furnish a large, flat field of view with 

 high power and yet without loss of light. For this purpose the curves of all our 

 lenses are ground by special formulae. The telescopes show objects right side 

 up, unless ordered otherwise. * 



The object-glass has a very large aperture, and is focussed by rack and pinion, t 

 but the eye-piece is focussed by simply turning its head to the right or left in an 

 improved screw-like manner. 



By a method of construction peculiar to ourselves, we are enabled to guarantee 

 the line of collimation correct for all distances without making use of the very objec- 

 tionable adjustment for the object-slide by means of inner rings, which time and 

 experience has proven to wear loose too readily, thus rendering this adjustment 

 worse than none at all. 



The eye-pieces are thoroughly achromatic, and their lenses are mounted in such 

 a perfect manner (a method also peculiar to us) as to require 110 further adjustment 

 with regard to the axis of telescope. 



*It should be remembered that the focal length of the object glass is limited in engineering instruments and 

 that a high power is obtained only at the sacrifice of light. To obtain the fullest satisfaction, telescopes intend- 

 ed for close work, as in stadia measurement, etc., should invariably be ordered to be inverting. The 

 brilliancy with which objects appear in such a telescope, owing to the amount of light gained by 

 saving two lenses in the e"-piece is vary marked as compared with one of the same power anH focal length 

 showing objects erect. 



tThis rack and pinion motion is now so placed upon our telescopes that it is more easy of access by either 

 hand than when placed at the side, as shown in most of our cuts. 



