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XVI. A very Simple and Accurate Cathetometer. 

 By F. L. O.Wadsworth*. 



OF the various standard physical instruments which are 

 usually found in a student's laboratory, the cathetometer 

 may justly be considered as one of the most instructive and 

 valuable, both because of the many principles involved in its 

 adjustment, and because of the number of measurements 

 which may be made with it. Unfortunately, good catheto- 

 meters (and it is never good policy to use poor instruments 

 for the purposes of instruction) are so expensive, as made at 

 present, that one or at most two are all that one laboratory- 

 can afford. For this reason it may perhaps be of interest to 

 briefly describe a form of cathetometer recently designed by 

 the writer which costs less than one-tenth as much as the 

 best German or English instruments, but which has shown 

 itself in use to be quite as accurate as and in some respects 

 even more convenient of manipulation than the latter. 



In the new arrangement, the general method of comparison 

 now employed in nearly all of the most accurate linear 

 measurements is followed : i. e.j the images of the observed 

 points and of the lines on a standard bar, placed parallel with 

 the length to be measured, are brought in succession into the 

 field of an observing telescope or microscope, and their 

 relative position determined by means of a micrometer or 

 some equivalent arrangement. In previous forms of catheto- 

 meter, this has been done by rotating the observing-telescope 

 itself on a long heavy- vertical axis; in the new form, all of these 

 heavy rotating parts are dispensed with, the observing-tele- 

 scope is fixed in position and the images of the object and 

 scale brought successively into the field by means of a light 

 silvered mirror mounted on a vertical axis just in front of the 

 objective. A sensitive level attached to the upper end of this 

 axis enables the latter to be set accurately vertical, this adjust- 

 ment being made as in an ordinary cathetometer. 



The complete arrangement is shown in elevation in fig. 1 

 and in plan in fig. 2. A, fig. 1, is the mirror frame and B is 

 the level, mounted at the lower and upper end respectively of 

 the short, conical axis C. The boss D, in which this axis turns, 

 is attached by means of a geometrical clamp at E to a split 

 cap which slips over the end of the observing-telescope T and 

 is clamped thereon by means of the screw F. This much 



* From the American Journal of Science, January 1896. 



