In November and December I tried my new double-image micrometer.

My double-image micrometer was much used for observations of circumpolar double stars.

A skilful workman, instructed by Mr Simms, easily reduced the error to about 2".5 (which would leave its mean error nearly 0), the western Y being moved to the north so far as to reduce the reading of the transit micrometer, when pointed to the south, from 35r.500 to 35r.000.

For observing the Transit of Venus of 1882, the general impression appears to be that it will be best to confine our observations to simple telescopic observations or micrometer observations at Ingress and Egress, if possible at places whose longitudes are known.

Many years ago a double-image micrometer, in which the images were formed by the double refraction of a sphere of quartz, was prepared by Mr Dollond for Capt. Smyth, R.N.

Adopting the same principle on a larger scale, I have had constructed by Mr Hilger a micrometer with double refraction of a sphere of Iceland spar.

The micrometer eye-piece used was substantially the same as Foucault's, except that part of the inclined plate of glass was silvered, thus securing a much greater quantity of light.

Thus the micrometer eye-piece is much simplified, and many possible sources of error are removed.

The first observation with the new micrometer eye-piece was made April 2, the deflection being 115 millimeters.

The Micrometer.

Manner of Using the Micrometer.

The fork was so placed that the light from the revolving mirror was reflected to a piece of plane glass, in front of the lens of the eye-piece of the micrometer, inclined at an angle of 45°, and thence to the eye.

The eye-piece of the micrometer was then set approximately and the revolving mirror started.

This was done by setting the eye-piece approximately to the expected deflection, and turning the whole micrometer about a vertical axis till the cross-hair bisected the circular field of light reflected from the revolving mirror.

The difference between the original reading and that of each measurement was noted, care being taken to regard the direction in which the screw was turned, and this gave the difference in length between the standard and each succesive portion of the steel tape in terms of turns of the micrometer-screw.

Determination of the Value of Micrometer.

d = deflection as read from micrometer.

The columns are sets of readings of the micrometer for the deflected image of slit.

The numbers contained in the columns headed "Position of Deflected Image," "Position of Slit," and displacement of image in divisions were obtained as described in the paragraph headed "Micrometer," page 120.

The "radius of measurement" is the distance between the front face of the revolving mirror and the cross-hair of the micrometer.

In the last two sets of June 13, the micrometer was fixed at 113.41 and 112.14 respectively.

Necessarily some of the groups include others: Electric light (1 set) 299850 Set micrometer counting oscillations (2) 299840

The correction for the brass scale would be +.0000105, or the whole correction for the micrometer would be +.000004.

The comparison of the micrometer with two scales made by Mr. Rogers, of the Harvard Observatory, has been completed.

They represent the value of one turn of the micrometer in millimeters.