48 G. J. BURCH ON A NEW MICROMETER. 



very accurate. All but the scale itself should be blackened. For 

 some very brightly lit objects a transparent scale is useful, while for 

 very faint ones, or for front light, a white scale on a black ground, or 

 lines engraved on a polished strip of German silver, and illuminated 

 by oblique light, may be employed. This last is the best for star 

 measurement with the telescope — the delicate silvery lines standing 

 out with peculiar distinctness, and without interfering with faint 

 stars. 



Advantages of the New Micrometer. 



It is easy to make. Any one who possesses a Beale's Neutral Tint 

 Reflector may adapt the New Micrometer to it. To adjust the scale 

 (which should be numbered with the figures reversed) , so that it may 

 read decimals of an inch or millimetre, &c, use a stage micrometer, set 

 the instrument in position, and slide the scale along the stem nearer 

 or further from the eye, as it may require, till on adjusting the focus 

 so that the apparent distance of the two images may coincide, every 

 10th division on the scale shall cover the l-100th or 1,000th of the 

 stage micrometer, according to the power used. Then mark the 

 position of the clip on the stem with a file. This position should be 

 found for each objective with each eye-piece ; the higher the power, 

 the more delicate is the measurement. 



To the optician, the construction of this Micrometer offers no 

 difficulties. There is no screw to make accurate in thread and in 

 bearing — no microscopical scale to engrave. It is made in the way 

 that is most convenient, and afterwards adjusted to the required 

 standard. 



It is easy to use. The Micrometer, with its scale clip set to the 

 mark corresponding to the power employed, has only to be slipped on 

 in place of the ej^e-piece cap, and the measurement made by inspec- 

 tion. If the object does not lie in the right direction, twist the clip 

 on the stem, and the image of the scale will revolve in the field of the 

 microscope, so that it can be set exactly across the object. If the 

 light is too strong to permit the scale to be seen, the lamp may be 

 shaded with one hand, while the stage and focussing is managed with 

 the other. Then there is no calculation required. The stem may 

 be marked to read fractions of an inch, a line, or a millimetre. 



As regards accuracy, I do not pretend that it will supersede the 

 Spider-line Micrometer. But as far as all scale micrometers are 

 concerned, it is not inferior to any. For it will fill the field with 



