G. J. BURCH ON A NEW MICROMETER. 47 



from the nearest limit of distinct vision to infinite distance. Then 

 why not dispense with the lens altogether, and compare the 

 reflection of the scale itself with the object, adjusting its distance 

 till the divisions on it coincide with those on the stage-micrometer ? 

 At first it seemed as if it would be hard to compare the reflection of 

 a real scale with an optical image, but it soon proved otherwise. 

 Just as in the camera lucida, you see the image of the object thrown on 

 the paper, and may measure it upon a scale drawn there, so here you 

 see the reflection of the scale in the field of the microscope, and may 

 compare it with the object. 



This, then, is the principle of the new Micrometer. Briefly, it is 

 an inverse application of the camera lucida, coupled with the direct 

 use of a scale, the graduations of which are adjusted to size by vary- 

 ing its distance from the eye-piece. 



In its simplest form, which I prefer, the instrument consists of a 

 cap, A, slipping over the eye-piece, and carrying an arm, B, on which 

 slides a clip, C, to hold the scale, D. The reflexion of this scale is 

 thrown into the eye by the diagonal glass, E, and compared with the 

 object seen at the same time through the eye-piece, F. (Plate IV.) 



It may be objected that this is a heavy side-strain for the tube. 

 Practically, I have not found it so, but the stem may be brought 

 parallel with the tube by using a second larger mirror of looking-glass 

 as in the second figure, or, the whole thing may be made smaller by 

 having a lens to form the image of the scale. But then the instru- 

 ment loses its simplicity, and is not so easy to use. 



The stem should be about 10 inches long, and should be made to 

 unscrew. Then, as to the reflector, I find thin cover glass gives a 

 double image. Sommering's steel disc, the camera lucida, or Dr. 

 Beale's neutral tint glass, answer equally well ; but for some pur- 

 poses I should give the preference to a piece of looking-glass with 

 the amalgam scraped off in the centre. If silvered glass is used, a 

 tiny drop of nitric acid from a capillary tube will eat the requisite 

 hole very cleanly. In this case the second image is completely 

 overpowered by the superior brilliancy of the reflection from the 

 silvered surface. 



The scale may be copied from any graduated rule — millimetres 

 are a convenient size — the length of the stem giving plenty of 

 latitude in this respect. It may be drawn on ivory or card. A 

 barometer scale, with its vernier, answers admirably — there being so 

 few divisions, the eye is not confused in counting them — and it is 



