THE COBWEB MICROMETER. 
45 
CHAPTER III. 
ACCESSORY APPARATUS. 
62. Micrometers. In examiningjobjects with the micro- 
scope, it is often desirable to ascertain their exact dimensions. 
Measuring instruments for this purpose, called micrometers, 
are sometimes applied to the object itself, but more frequently 
to the magnified image. Both classes of micrometers have 
their uses, for which they are specially adapted, and the pur- 
chaser can select that form which is best suited to the investi- 
gations for which it is to be used. 
63. Glass Stage Micrometers are furnished mounted in 
brass, with lines ruled from fo x^o of an inch. This instru- 
ment is chiefiy used for determining the value of measurements 
made by the various eye-piece micrometers. 
64. Tlie Cobweb Micrometer was invented by Rams- 
den for telescopes, but it is equally applicable to the micro- 
scope, and, until lately, has been considered superior to all 
others. Ramsden employed the positive eye-piece, but the 
negative eye-piece is equally available, and for use with the 
microscope it is to be preferred. 
The Cobweb micrometer is shown in Figs. 18 and 19, and 
consists of a negative eye-piece, in the focus of which two 
cobweb threads are stretched across the field ; one of these 
threads can be separated from the other by a screw having 
about fifty threads to the inch. The head of this screw, shown 
at Fig. 18, is divided into one hundred parts. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
