Engineers' 

 Instruments and Their Adjustments. 



Written especially for this Catalogue by Dr. LEONARD WALDO. 



General Remarks. 



THE OPTICAL PART. 



in the .construction of telescopes for engineers' instruments, several difficulties 

 present themselves. To be portable, the telescope must be of small aperture, and 

 of short focus. To make it of short focus and yet retain sufficient aperture to give 

 the light necessary with the eye-pieces used, requires especial care on the part of 

 the maker, both in securing the true curves for the crown and flint glass lenses, 

 which make up the achromatic object-glass, and in adapting an eye-piece which 

 will secure a flat field, with the least distortion. 



Of the many forms of eye-pieces known, Messrs. C. L. BERGKR & SONS, after care- 

 ful experiments with the formulas suggested by the distinguished astronomer, Sir 

 George B. Airy, and the late Mr. Kellner, of Wetzlar, (the two best formulas 

 known,) have adopted the latter. Mr. Kellner's formula employs four lenses, 

 mounted separately, and so arranged as to secure a flat field of the sharpest defini- 

 tion, to the very edge. 



The magnifying power of the telescope depends upon the relation between the 

 focal length of the object-glass and the focal length of the eye-piece, considered as 

 a single lens : Thus 



K F = focal length of the object-glass, 



f = " k * " eye-piece, 



TR1 



Then -^- = magnifying power of telescope. 



It is readily seen that the magnifying power may be increased or diminished by 

 altering the focal length f , of the eye-piece ; but if the maker increases the power 

 too much, since only a fixed amount of light can enter the object-glass, this fixed 

 amount of light is spread over too much surface in the field of view, and the object 

 seen is therefore too faint. If the maker gets the magnifying power too small, 

 then the engineer has a difficulty in pointing the telescope accurately. Some other 

 points in regard to the magnifying power will be referred to in the description of 

 the transit telescope. Messrs. C. L. BERGEB & SONS, have found about twenty-four 

 diameters to be the most satisfactory power for their Engineers' Transit Telescope; 

 and for levels the powers increase in proportion to the size of the instruments. 



Very much depends upon the optical part of any instrument, and very little has 

 been put into the hands of the practical engineer by which he may rigidly test it. 

 TJie following suggestions may be found convenient. 



The telescope should come sharply into focus, and a very little movement of the 

 focussing screw, either way, should cause the image to blur. When it is sharply 

 focussed, covering any part of the object-glass without altering the focus, should 

 not alter the sharpness of definition but merely cut off light. The pencil of light 

 which enters the object-glass, should come out at the eye end. To ascertain this, 

 see whether a pointer which you place just in contact with the edge of the object- 

 glass, can be wholly seen in the small disc of light which you will notice at the 

 small opening of the eye end when you draw your head back some inches from the 

 telescope, and point the telescope towards the sky. If the pointer cannot be seen 

 up to the very edge, then the maker has inserted a diaphragm which cuts off light 

 from the object-glass, and, very probably, to conceal the faults in making. In this 



