1877.] -~ The Study of Zoclogy in Germany. 393 
the microscope is not sufficient to enable us to do this, but we are 
obliged in every case to subject the preparation to a special treat- 
ment. ‘The first thing to be done is to make the object trans- 
parent enough to let the light pass through it to the objective, 
which is usually done by mounting it in glycerine or in Canada 
balsam, both of which substances have a high index of refraction, 
and therefore when they penetrate the interstices of a tissue do 
away with the refraction inside of it, so to speak; for in every 
tissue the different parts refract the light so variously that a ray 
passing through frequently changes its path, thus confusing the 
final image which reaches the observer’s eye. A layer of pow- 
dered glass lets the light pass through, but nothing distinct can 
be seen; if, however, the whole is immersed in Canada balsam, it 
immediately becomes beautifully transparent, because the balsam 
fills up the spaces between the bits of glass, and since balsam and 
glass refract light to about the same degree the mass becomes 
optically nearly uniform, and a ray of light can pass through it 
without being deviated from its course or destroying the image. 
The action on the tissues is identical, — and this should be care- 
fully remembered, because. balsam renders objects more trans- 
parent than does glycerine, so that in some cases one liquid is 
better than the other. It is a sign of inexperience to assert that 
balsam is better than glycerine, or vice versa, for they are both 
useful, but for different purposes. 
Tn order to observe the cells well it is necessary not to have 
too many superposed layers in the field of view, but to make 
the object as thin as possible. This is usually accomplished by 
_ Making sections. So important and so useful are such very thin 
Slices that probably nine tenths of every histological collection 
Consist of them. The first thing, therefore, is to acquire skill in 
making sections, and the perfection reached will mainly decide 
how far the progress of the student shall continue. The im- 
portance and benefits of making sections have led to the inven- 
tion of a great many mechanical contrivances for cutting them. 
One form of cutter or microtome well adapted to its object was 
described in the April number of the NATURALIST of this year. 
Numerous other forms have been suggested, but those with which 
Tam acquainted all have some defects. Free-hand cutting still — 
 Temains absolutely indispensable. It may be acquired by pa- — 
: tient practice even by those who have no special manual skill, 
Just as we are all able to write. There are many things which | 
cannot be cut with a machine. The razor for cutting should be 
