56 
ACCESSOKY APPARATUS. 
by the number of turns and parts of a turn given to the screw to 
cause the image of the object to pass entirely across the cobweb. 
For using this micrometer, the stage (if movable) is made 
fast by a clamp. The object-slide is placed upon the plate A 
A, which is rotated till the object is in a proper position to be 
measured. One edge of the object is brought to coincide 
exactly with the cobweb in the field of view, and the reading 
of the scale v, graduated head y, and vernier n, are accurately 
noted, or if the object is a small one, the graduated head y can 
at once be set to zero of its scale. The milled head is then 
turned till the object has passed entirely across the line in the 
eye -piece, and the readings of the scale, graduated head, and 
vernier are again examined. The difference between the first 
and second readings is the exact measurement of the object. 
The great accuracy of this instrument, and the facility with 
which it can be used on any microscope, no preliminary calcu- 
lations being required, will commend it to the favor of all who 
desire the most perfect micrometer yet invented. 
INSTRUMENTS FOR DRAWING WITH THE MICROSCOPE. 
74. Wollaston’s Camera Ciicida is the instrument com- 
monly used by artists for sketching from nature. Tliis instru- 
ment is fitted to the eye-piece of the microscope, and enables the 
observer to sketch upon paper, placed on the table, the magni- 
fied image of any object seen in the microscope. It consists of 
a four sided prism of glass, having its faces and angles so 
arranged that light entering the first face of the prism is totally 
refiected by the second and third faces, and emerges perpen- 
dicular to the fourth face of the prism, and at right angles with 
its original direction. 
When using this camera^ the microscope is placed in a hori- 
zontal position, and the object appears projected upon the 
paper placed on the table to receive it, and it may be traced by 
a pencil which the eye sees at the same time as the object. 
75. Wacliet’s Camera I^tieida. This instrument consists 
of a triangular prism, having its three faces and angles equal. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
