390 



THE MICEOSCOPE. 



examined, is secured in a black silk bag ; and this is fastened to a plate 

 of brass, termed the frog-plate, shown at a a a in fig. 183 : this should 

 be contrived so as to be held firmly by some part of the stage of the 

 microscope, and permit of its being moved about with it. Although 

 the shape of the plate differs with every maker, the mode of using it, 

 nevertheless, is nearly the same in all. The bag provided should be 

 about three or four inches in length, and two and a half inches broad, 

 as shown at b b, having a piece of tape, c c, sewn to each side, about 

 midway between the mouth and the bottom ; and the mouth itself 

 capable of being closed by a drawing-in string, d d. Into this bag the 

 frog is placed, and only the leg which is about to be examined kept 

 out of the mouth ; the string d d is then to be drawn so tight around 

 the small part of the leg, as to prevent the foot from being pulled into 



fig. 183. 



the bag, but not to stop the circulation ; three short pieces of thread, 

 fff, are now to be passed around the three principal toes ; and the bag 

 with the frog is to be fastened to the plate a a by means of the tapes 

 c c. When this is accomplished, the threads ///are to be passed either 

 through some of the holes in the edge of the plate, three of which are 

 shown at g g g, in order to keep the web open ; or what answers better 

 is a series of pegs of the shape represented by h, each having a slit i 

 extending more than half-way down it ; the threads are wound round 

 these two or three times, and then the end is secured by putting it 

 into the slit i. The plate is now ready to be adapted to the stage of 

 the microscope : the square hole over which the foot is placed must be 

 brought over the aperture in the stage through which the light passes 

 to the object-glass, so that the web may be strongly illuminated by the 

 mirror. The power required to view the circulation will be a one-inch 



