386 GrEUBB — Some Ncio Forms of Geodetical Instruments. 



At right angles to this tube is mounted a smaller tube carrying at its outer end 

 a diaphragm (J), preferably of glass coated with some opaque material, through 

 which lines are cut i-epresenting a cross, star, circle, or any other desired device. 



At the base of this same tube, near its junction with the main or sighting 

 tube, is placed an achromatic lens, the distance between the diaphragm and the 

 achromatic lens being equal to the principal focus of that lens ; consequently rays 

 of light from the sky or any other source of light which pass through the trans- 

 parent portion of the diaphragm, diverge until they reach this object-glass (o), 

 and are by it rendered parallel, and are reflected by the diagonal plate or plates, 

 jyp, once again as parallel rays, into the eye of the observer ; the result being that 

 the observer sees, superposed upon the object he is aiming at, an image (generally 

 called a "virtual" image) of the cross or device cut upon the diaphragm; and 

 inasmuch as the arrangement, when properly adjusted, is such that the rays from 

 the diaphragm enter the eye under exactly the same conditions as if from the 

 distant object, the cross appears not only superposed on the object, but at the 

 same distance as the object itself. As a consequence of this, the cross is seen 

 absolutely sharp with the same focussing of the eye as that necessary for viewing 

 the distant object, and there is no straining of the eye to see both in focus at the 

 same time ; also it follows that there is no pai'allax, that is to say, that the cross 

 and the object aimed at, if made to coincide when the eye is in the centre of the 

 tube, will equally well coincide no matter what portion of the sighting tube the 

 eye is placed opposite to ; in other words, there is no necessity for the observer 

 to keep his eye in any fixed position. 



It will be noticed in this instrument that three plates of glass are shown 

 superposed upon one another for reflecting tlie image of the device on the 

 diaphragm into the eye : — The object of this is to intensify the brilliancy of the 

 reflected image without sensibly diminishing the apparent brilliancy of the object 

 aimed at. Later on, however, it was found that a more practical plan of increas- 

 ing this brilliancy was to use one single piece of glass, and coat this with a semi- 

 transparent and highly reflective film. A long series of experiments carried out 

 by Professor J. Emerson Reynolds, F.R.S., and Mr. G. Rudolf Grubb, B.A.I., 

 resulted in a modification of a process invented by the former by which the desired 

 film was obtained. 



It will be seen that there are two distinct principles involved in this instrument. 



(1). The complete absence of parallax is obtained by the employment of the 

 collimating lens " 0," which brings into parallelism the raj^s jiroceeding from the 

 small cross at " D," so that these i-ajs, enter the eye under exactly the same con- 

 ditions as they would if proceeding from a large cross at a great distance, instead 

 of a small cross at a small distance. 



(2). The superposition of this image of the cross and that of the object is 

 obtained by the use of the semi-transparent and semi-reflective film chemically 



