TRANSACTIONS OF THE SECTIONS. 21 
magnifying power, 2. e. the distance between the two lenses, until the four stars 
appear equidistant. Ifthe power be too small, the middle space will be greater than 
that between each pair of stars ; if too great, the reverse. When the four are, how- 
ever, satisfactorily adjusted and the scale read off, the measured angular distance is 
equal to the constant angle of the crystal divided by the magnifying power employed. 
By placing the four stars at equal intervals, the double distance will be measured, 
and the uncertainty of a contact between two perhaps very unequally bright stars 
avoided, which is highly desirable. It is therefore of the utmost importance to know 
the constant angles of the prisms, also the limits of magnifying power of the variable 
eyepiece, with all possible accuracy. It does not enter our purpose here to discuss 
the various methods recommended by Dr. Pearson for determining these instrumental 
constants ; suffice it to remark, that when once found they are not liable to change, 
and their investigation is by no means difficult. 
The observation of position angle, though less certain than that of distance, is 
at least equal to what can be made with any other kind of micrometer. For this 
purpose a fine diametral line is cut across the flat side of the field lens, which be- 
comes visible when screwed into the focus of the eye-lens. This line must be ad- 
justed by running the double star, or if more convenient any adjacent bright one, 
along its entire length.. This done, the prism, which is attached to the vernier of the 
position circle, must be moved round until the two images of the line are seen in co- 
incidence, when the reading will be the zero-point of the position circle. For these 
two operations the field requires illumination, though not for the absolute measures. 
The four stars seen through the prism must next be brought into the same straight 
line, as in the measure of distance, when the difference between the circle-reading 
and the zero previously found, plus or minus ninety degrees, will give the angle. of 
position. Unless the equatorial adjustments of the telescope have been very cor- 
rectly made, it is better to take half the number of zero readings before, and half 
after those of position, so as to eliminate any change of the zero arising from the 
alteration of hour-angle during the observation. 
A temporary defect in our micrometer, which could not be remedied without part- 
ing with it just at the time most convenient for its use, prevented the observations 
of angle of position being made. A fewonly were attempted, but sufficient to show 
that good results may be expected in this coordinate also when the micrometer is 
put into working order. But as the observations appended are to be regarded merely 
as examples of its capabilities as a distance-measurer, under unfavourable conditions, 
and by no means in the light of a series of scientific results, it would not be render- 
ing justice either to the instrumentor to the observer to add the few imperfect angles 
of position obtained, and they have therefore been omitted. 
No kind of micrometer is so suitable to, or convenient for, an unprofessional 
astronomer, who cannot enjoy the luxuries of an observatory ; and it is no exaggera- 
tion of its merits to say, that any good portable telescope, so equipped, will enable 
its possessor to compete successfully with far larger instruments in fixed observatories 
not so provided. To prove this point, it is only necessary to refer to the annexed 
observations of well-known double stars, measured on different nights, frequently 
with different crystals, and therefore perfectly free from any pre-occupation of mind. 
As first attempts with a strange instrument, they are naturally open to great im- 
provement ; but with practice and care, it seems reasonable to hope that differences 
exceeding half a second, or for small distances a twentieth part of the measured 
space, will rarely, if ever occur. 
It is a striking proof of the excellence of the Smythian telescope, that with an 
aperture of only 3°6 inches, with the light diminished by the duplication of the image, 
and by its transmission through tolerably thick prisms of rock-crystal, powers over 
200 were so readily usable. Its definition is beyond all praise : without the prisms, 
it bears a power of 400 well, and separates the closer double stars in a vianner 
which puts to shame many much larger telescopes. 
