THE YOUNG SCIENTIST. 
119 
\ acid; 3 ounces water. The pyrogallic acid 
! and the citric acid are dissolved in water 
I when the solution is ready for use. This 
solution does not keep very well, and 
should always be prepared fresh. This 
I developer requires a longer exposure than 
the iron developer, and is more appro- 
I priate for negatives than for positives. 
i The quantity of citric acid can be modified 
II to the same circumstances which regulate 
the treatment with acetic acid in the iron 
developer. 
1 In making either positives on glass, or 
I ferrotypes, it is desirable to have a white, 
metallic-looking film. The addition of one 
drop of nitric acid, 10 grains nitrate of 
potassa, and a few drops of silver solution 
it to others. Of the many forms of appar- 
atus specially designed for holding the frog 
and displaying its foot, the common bras& 
frog-plate has been most generally used, 
but it has this disadvantage, that after the 
frog has been properly secured, it is almost 
impossible to move the plate under the 
clips, which, in the cheaper microscopes, 
serve to retain the object in place on the 
stage. A very simple addition to the ordi- 
nary frog-plate serves to remedy this defect, 
and enables the observer to bring under 
view any part of the web of the foot. 
This frog-plate consists of two plates 
firmly rivetted together at one end, with a 
strip between them which holds them a 
little more than an eighth of an inch apart. 
to the first formula, will accomplish the de- 
sired end. 
The developer renders the latent picture 
upon the collodion film visible. It pro- 
duces a decomposition of the silver solution 
in such a manner as to leave the metal in a 
reguline condition only on those places 
which have been acted upon by light. 
A New Form of Frog-Plate. 
rpHE circulation of the blood in the foot 
\ of the frog has always been a favorite 
' subject for exhibition. The subject is so 
easily prepared, and it may be seen by such 
Ij ordinary instruments, that few persons who 
j own a microscope need be deterred from 
I attempting to see it themselves, or to show 
The under plate is plain, with a hole which 
corresponds with the hole over which the 
web of the foot is stretched. This lower 
plate passes under the clips of the stage^ 
which retain it securely, but allow perfect 
freedom of motion. The frog, after being 
swathed in a strip of moistened muslin, is 
tied to the upper plate, the notches at the 
side of which hold the windings of the 
string from sliding. Around the large hole 
over which the web is spread, there are 
several holes, the object of which is as fol- 
lows : After the frog has been tied to the 
plate, its position being so regulated that 
the web of the foot will come fairly over the 
hole, the operator takes three pieces of 
thread, each having a slip-knot at the end» 
These slip-knots are passed over the toes 
and drawn tight; the free ends are passed 
through such of the small holes as are best 
