JACKSON S MICROMETER. 
47 
ascertained to the hundredth of a turn of the screw, and as the 
screw has fifty threads to an inch, it follows that the magnified 
image of an object may be measured to the five-thousandth of 
an inch. 
With an object-glass of one-eighth of an inch focus, the im- 
age, formed at the focus of the negative eye-piece, where the mi- 
crometer threads are placed, will be magnified about fifty diam- 
eters without the power of the eye-lens ; it follows, therefore, 
that a quantity as small as the two-hundred-and-fifty-thousandth 
of an inch should be appreciable by such an instrument, but 
in practice this has been found impossible, as no achromatic 
power has yet been made, capable of separating lines so 
close as the one-hundred-thousandth of an inch. Great care is 
requisite in ascertaining the value of the measurements made 
by this micrometer, to avoid several species of error which 
will be pointed out in section 71. The measurements by this 
micrometer are not as delicate as they appear to be, but are 
the most reliable that can be obtained by any eye-piece mi- 
crometer. 
65. Rosses Eye-Piece Micrometer consists of a circle of 
thin glass, ruled with micrometer lines, set in a brass ring, and 
screwed into the lower end of the positive eye-piece, so as to be 
seen exactly in its focus. To adj ust the focus to suit difierent 
eyes, the ring may be screwed up a little nearer to the front 
lens, or adjusted a little more distant. This is a very conven- 
ient form of micrometer for use with low powers. 
66. Jackson’s Micrometer. Mr. George Jackson, in 1840, 
invented another form of micrometer, which, with the improve- 
ments suggested by experience, he thus describes : 
Short bold lines are ruled on a piece of glass; and, to facil- 
itate counting, the fifth is drawn longer, and the tenth still long- 
er, as in the common rule. Yery finely levigated plumbago is 
rubbed into the lines to render them visible, and they are cov- 
ered with a piece of thin glass, cemented by Canada balsam^ 
to secure the plumbago from being rubbed out.” 
The slip of glass thus prepared is placed in a thin brass frame, 
as shown in Fig. 20, so that it may slide freely, and is acted on 
at one end by a pushing screw, and at the other by a slight 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
