Wadsworth — Simple and Accurate Cathetometer. 



41 



Aet. X. — A Very Simple and Accurate Cathetometer / 

 by F. L. O. Wadsworth. (With Plate I.) 



Or 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 cathetometers (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 in- 

 struments, but which has shown itself in use to be quite as 

 accurate and in some respects even more convenient of manip- 

 ulation than the latter. 



In the new arrangement, the general method of comparison, 

 now employed in nearly all of the most accurate linear meas- 

 urements, is followed : i. e., 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 posi- 

 tion determined by means of a micrometer or some equivalent 

 arrangement. In previous forms of cathetometer this has been 



done by rotating the observing telescope itself on a long 

 heavy-vertical axis ; in the new form, all of these heavy rotat- 

 ing parts are dispensed with, the observing telescope is fixed in 

 position and the images of the object and scale brought succes- 

 sively into the field by means of a light silvered mirror 

 mounted on a vertical axis just in front of the objective. A 



