G. J. RURCH ON A NEW MICROMETER. 



49 



divisions as fine and as close as can be distinguished at ten inches, 

 and no scale can do more. But it is hard to get an eye-piece Micro- 

 meter that reads exactly thousandths, and even then it is only with 

 one power. 



The Aerial Stage Micrometer, like this one, has an adjustible 

 scale, but on account of the lens being used to produce the image, 

 any error in the setting will make five or six times the difference 

 that it would in mine — in which the error of measurement is the 

 same fraction of the object that the error of setting is of the length 

 of the stem used. The personal equation, too, is evidently elimi- 

 nated if the microscopist himself adjusts the setting by an accurate 

 stage micrometer. But its principal advantage is found in the 

 number of uses to which it may be put. When the stem is unscrewed 

 it leaves the camera lucida intact. The clip will carry a Goniometer 

 — simply a black index, like a watch-hand, with parallel sides, re- 

 volved by a button from the back in a graduated semicircle — the 

 diameter being drawn specially broad for distinctness. It is as 

 easy of application to a hand-magnifier as it is to a microscope. I 

 use it with my dissecting lens, and even with my diatom-finder — a 

 Coddington, of l-20th of an inch focus. For more than a year I 

 have had one fitted to my small lathe, and have rejected the ordinary 

 gauge in its favour. The metal seems to melt away under the tool 

 by thousandths at a time, down to the required size on the scale. 

 My experience of it is that it would be invaluable to the optician or 

 watchmaker ; it reduces the chance of going too far, and saves a 

 deal of time. Then with the Spectroscope it is quite as easy of 

 application, and as comfortable to use as Browning's arrangement. 



I have already mentioned the scale engraved on silver and illu- 

 minated by oblique light, as being best for astronomical use. It is 

 sj^ecially adapted for star-spectra — a slight shading of the dark 

 lantern reducing the brilliancy of the lines to any extent without 

 impairing their distinctness. 



Nor is this all. Not only can this Micrometer be applied to any 

 and every kind of optical instrument, but it can be used by itself for 

 many things that neither carpenter's rule nor callipers can reach. 

 You have only to fit a rod ten inches long in place of the microscope 

 tube, and set the scale ten inches from the reflector, and you may 

 measure the size of anything in the plane of the end of the rod. 



The internal diameter of a soda-water bottle — the contraction of 

 the iris in a strong light — the diameter of a dew-drop — or the sweep 



