636 SEC. 15. GEOGRAPHY. 



3109b. Metroscope. For the determination of dimensions 

 of distant objects (a drawing). 



Dr. Snellen, Physiological Laboratory and Opht halt no- 

 logical School, Utrecht. 



Before the objective of a telescope are placed two mirrors, one above the 

 other, each occupying one half of the field of view, and inclined to the plane 

 of the objective at an angle of -1, the angle between them being 90. On 

 a cross bar, at each side, a mirror parallel to the first is moveable. Looking 

 through the telescope, by means of these two sets of mirrors, two objects are 

 seen straight above each other, and the mutual distance of these objects 

 must be, at whatever distance they are from the observer, equal to the dis- 

 tance of the two outer mirrors. (Handbuch der Ophthalmologie, von Graefe 

 und Saemisch, III. p. 203.) 



3110. Level, with independent bubble, Gravet's system, with 

 stand. M. Tavernier Gravet, Paris. 



3111. Portable Repeating Circle (14 centimetres). 



T. $ A. Molteni, Paris. 



3112. Patent Surveying Level. Francis Pastorelli. 



This instrument combines several important improvements, including 

 increased facility in use, greater steadiness and freedom from vibration and 

 possibility of derangement, accurate adjustments, and great durability. 



Tripod und Staff Head. Stability is of the utmost importance. This is 

 secured in the staff head (which is the adoption of a plan by Wm. Froude, 

 Esq., C.E.), which has the checks set wider apart, the leg joints being similar 

 to an inverted mortar with strong trunnions, which can be tightened in their 

 bisected cylindrical bearings by means of capstan headed screws. As -wear 

 goes on they can always be kept perfect ; this cannot be done with the ordi- 

 nary staff head. 



Ball Joint and Clamp. The instrument is free to move with 20 of inclina- 

 tion. This is most important, as much valuable time is saved, more especially 

 upon hilly ground, as it can be almost set instantaneously. 



Adjustments. The parallel screws work in movable hemispherical nuts, 

 which are held in seats in the parallel plates, those in the lower being held in 

 brackets. Their action, permits the upper plate to be worked at an inclination 

 of about 15 without their being strained or twisted. By their means the 

 upper parallel plate is made to clamp or set free the inverted cup on the 

 vertical axis of the instrument ; in addition, they cause it to heel over to per- 

 fect the adjustment, so as to bring the main bubble in the centre of its run 

 whenever a force is applied to them greater than is necessary to clamp the 

 inverted cup. 



Suspension of Telescope. The telescope is solidly fixed to its base, which 

 is parallel to the axis of it ; it has a female screw with a true surface which 

 fits on to the gun metal centre, so that when the instrument is reversed in any 

 direction the main and circular bubble is retained in the centre of their run. 

 Around the telescope are two gun-metal collars, accurately turned and ground 

 to a perfect circumference ; by their means are adjusted the mechanical and 

 optical axis of the telescope and the line of collimation ; the main bubble is 

 dead fitted ; it neither admits nor requires adjustment, as it is done in con- 

 struction. 



Diaphragm. The diaphragm with its collimating screws cannot be dis- 

 turbed. A grooved channel is formed in the eye end of the telescope, into 

 which they are sunk, and a cylindrical ring conceals them from sight, thus keep- 

 ing'them intact. This is one of the important adjustments of the instrument. 



