20 “ey! REPORT—1858. 
accurate results when applied to the measurement of small distances, that it appears 
singular, as well as a matter of regret, it should so long have awaited a fair trial, and 
the publicity it so well deserves. 
Although copiously described by Dr. Pearson in his valuable work on ‘ Practical 
Astronomy,’ so many varieties of double-image and other micrometers are there 
treated, that the one under consideration has :failed to attract more than a passing 
notice from the Doctor’s readers, and unfortunately no published results have made 
their appearance from which amateurs might judge of the accuracy attainable by its 
use. Had such been forthcoming, doubtless the Ocular Crystal Micrometer would 
have been generally adopted, and the well-grounded complaint, of the inferiority 
of the measures of distance of double stars to those of the angles of position, would 
never have been heard of. 
Perhaps the best method of drawing attention to this micrometer will be briefly 
to explain the construction and use of one, originally made for Admiral Smyth,— 
transferred by him to the Hartwell Observatory in 1829, on becoming possessed of 
the larger one described in his ‘ Celestial Cycle,’ and recently placed in the hands 
of Mr. Pogson for trial, by the kindness of its present owner, Dr. Lee. 
The Ocular Crystal Micrometer consists of a variable eyepiece, 7. e. one in which 
the second or field-lens is moveable by a rack-work, so as to vary the distance 
between itand the eye-lens. This distance is read off, by the help of a vernier, on a 
scale of equal parts, and this, as will be shown, is an important element in the ob- 
servation. By a well-known optical formula, if e, f, and O represent respectively 
the focal lengths of the eye-lens, field-lens, and object-glass of a telescope, when 
used with a certain eyepiece, also d, the distance between the first-named lenses, the 
magnifying power of the telescope with such eyepiece will be thus found :— 
O 
Power=| pe+t— d). 
From this it is manifest, that when the two lenses are in contact, the power will be 
a maximum; that as the distance between them increases, the power will diminish ; 
and that the equal divisions on the scale which records this distance will, when mul- 
tiplied by the factor 2, give the corresponding changes in the magnifying power. 
It is therefore sufficient to determine, with a dynameter, the magnifying powers 
when the lenses are in contact, and when most widely separated, and by simple pro- 
portion to tabulate the intermediate divisions and corresponding powers. By having 
two or three eye-lenses, which can be slipped (not screwed) into their cells, the 
range of powers is very considerably increased. Thus in our micrometer, eye-lens 
No. 1 extends from powers 261 to 134; No. 2, from 135 to 78; and No. 3 from 
71 to 48. 
To produce a double image, two prisms of rock-crystal—the one cut in the direc- 
tion of its optical axis, the other transversely thereto—are cemented together, so as 
to form an achromatic solid of double refraction. Six such prismatic solids, of con- 
stant angles from 2474” to 192”, are in our micrometer fitted into brass caps, which 
are made to slip on, in front of the eye-lens; so that if the separation of the images 
is too great, the prism can be immediately changed for another of a less constant 
angle, and vice versd. The double image is therefore formed after the telescope 
has performed its office, and is much cleaner and more distinct than in the original 
contrivance of Rochon, the inventor, who placed his prisms between the eyepiece 
and the object-glass. No wings or coronz trouble the observer as with divided eye- 
glass micrometers ; and although of course half the light is lost by the duplication of 
the image, the tedious additions of clock motion and illumination of the field are 
dispensed with. Indeed, the loss of light sustained by the use of illumination 
barely sufficient to render the epider lines of a wire-micrometer visible, far exceeds 
that occasioned by the transmission of the rays through good prisms of rock-crystal. 
Perhaps the whole secret of the excellence of this micrometer lies in the position of 
the crystal being BEFORE instead of behind the eyepiece. 
On viewing a double star through the Ocular Crystal Micrometer, two pairs will 
be seen, the four members of which must be brought into the same straight line 
by turning the crystal round in its cell. The measure is then made by varying the . 
