48 
ACCESSORY APPARATUS. 
spring. This is inserted in the focus of the eje-lens in the neg- 
ative eje-piece, through slits cut in each side for the purpose. 
The cell of the eye glass should have a longer screw than usual 
to admit adjustment for different eyes. When the frame is not 
employed, an inner piece of tube is drawn across the slits to pre- 
vent dust from getting between the glasses. 
Fig. 20. 
To use this micrometer, the object is brought to the centre 
of the field, and the coincidence between one side of it, and one 
of the long lines, is made with great accuracy, by means of the 
small pushing screw that moves the slip of glass ; the divisions 
are then read off as easily as the inches and tenths on a com- 
mon rule. The value of the divisions must, however, be accu- 
rately ascertained for each object-glass, in the same manner as 
for other eye-piece micrometers. The operation of measure- 
ment is then nothing more than laying a rule across the body to 
be measured ; and it matters not whether the object be trans- 
parent or opaque, mounted or not mounted ; if its edges can 
be distinctly seen, its diameter can be taken. By revolving 
the eye-piece, similar measurements can be taken across any 
other diameter. 
A similar micrometer slide, with its pushing screw, is attach- 
ed to the positive eye-piece, when this is preferred. 
67. Comparative Merits of Micrometers. Of these mi- 
crometers, the one shown in figures 18 and 19 is the most accu- 
rate and reliable. Its measurements are all made between two 
lines, neither of which is included in the measurement, the clear 
space between the lines being alone reckoned. As the neg- 
ative eye-piece is used, the object is clearly defined, free from 
all aberration. The expense of the Cobweb Micrometer alone 
prevents its general use. 
In Mr. Boss’s micrometer, the use of the positive eye-piece 
J. & W. GRUNOW & GO’S ILLUSTRATED 
