136 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 

 A KEW SPECTROSCOPE-MICKOMETER. 



Professor Rood, of New York, has devised a very conven- 

 ient eye-piece micrometer for use in spectroscopic measure- 

 ments. A thin, smooth, semicircular plate of silver is black- 

 ened by smoking it, the soot being attached by subse- 

 quent flowing with weak spirit varnish. On this dead black 

 surface, and perpendicular to its diametrical edge, lines 0.25 

 millimeter apart are ruled with a dividing engine, the num- 

 bers being afterward added. The opaque plate, thus pre- 

 pared, is placed in the interior of a negative, or, preferably, 

 in front of a positive eye-piece, so that it is in focus, and oc- 

 cupies nearly half the field. A lateral opening in the eye- 

 piece, somewhat nearer the eye, admits the light necessary 

 for illuminating the ruled lines. In general the difl*used light 

 of the room is sutticient for this purpose ; but, if not, a distant 

 lamp conveniently placed accomplishes the same purpose. 

 In this way a set of bright lines is seen in the field of view, 

 more or less bright as the lateral opening is more or less 

 shaded, which may be used with great satisfaction in fixing 

 the position of lines in spectra given either by prisms or 

 ruled plates. 4 1>, July^ 1873, 44. 



A NEW DOUBLE-IMAGE MICROMETER. 



A proposition has been made to the Paris Academy of Sci- 

 ence by a M. Noel, describing a proposed new form for the 

 double-image micrometer, which seems to have some advan- 

 tages over the divided object-glass and the divided ocular. 

 Noel places within and near to the principal focus of the tel- 

 escope a plane mirror, so adjusted that the image of the ob- 

 ject to be measured is formed at the side of the tube. The 

 mirror, however, is not of one piece, but is divided into two 

 separate halves ; the optical axis of the telescope and the line 

 of bisection are in the same plane, and perpendicular to this 

 plane is an axis about which either (or one) of the mirrors 

 may be revolved. The two images of the object, as formed 

 by the respective plane mirrors, may now be separated as in 

 the double-image micrometer, the degree of their separation 

 being equal to the angle included between the two planes. 

 The advantages of Noel's construction are (1) that the mi- 

 crometer screw is replaced by the divided circle, and (2) that 



