MACHINE FOR CU'ITING CIRCLES OF THIN GLASS. 
67 
to its centre, and so cut as to act as a spring when inserted in 
the opening of the stage. An axis extends horizontally from 
one side of the brass plate above described, to which is attach- 
ed a plate of wood about three inches square, on which the 
frog is secured in a bag. This plate carrying the frog, can be 
rotated so as to give to the animal the most easy position for 
extending the foot, so as to allow the web to be spread out upon 
a glass plate over the opening in the stage of the microscope. 
Oil the opposite side of the brass plate are sliding sockets, in 
which are inserted pivots or keys like those which tighten the 
strings of a violin. To these pivots or keys are attached the 
threads which extend the toes of the frog. The threads may 
be tightened by turning the keys, or their position changed by 
moving the sliding sockets. This instrument enables one to 
view the circulation in the frog with great facility. 
92. ITlacliiiie for Cutting Circles of Tliiii Glass. The 
base of this instrument, which is of mahogany, supports a strong 
bent arm of japanned cast iron. From the end of this arm, 
firmly attached to it, projects downward a cylindrical guide, 
two inches long, designed to steady the other parts of the ap- 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
