WILLIAM B. DWIGHT. 65 
probably also to some extent, of lithological specimens, 
as generally practised. Doubtless there are scientists to 
whose work in this line these remarks will not apply ; 
but there is reason to think that they are exceptions. 
The first erroneous tendency is the supposition that 
- the usefulness of a section will be very much in propor- 
tion to its thinness ; that is, ‘‘the thinner, the better.’’ 
To follow any such imperative rule is calamitous, for the 
following reason :—the object of slicing specimens is to 
obtain thereby as complete a knowledge as possible of 
every detail of the internal structure. In most objects of 
paleontological interest, and perhaps in many of litho- 
logical study, there will be details of coarser structure 
intermixed with details of exceedingly delicate structure. 
There may also. be details of delicacy intermediate be- 
tween these two. It is only the amount of contrast 
between the light and shades of the details in the finished 
section which will develop the structure. If a certain 
tissue is exceedingly delicate, it may be possible, at a 
certain rather thick stage of the thinning of the section, 
for this tissue or structure to be thin enough to admit 
light and yet thick enough to show itself by contrast 
with the surrounding ‘‘ filling’’ of the specimen ; where- 
as a little more thinning of the whole may, and probably 
will, so thin this particular tissue that there is not 
enough of its thickness left to present any contrast with 
its surroundings, and the knowledge of its presence wiil 
be entirely lost. Yet at this thicker stage of the section, 
the coarser portions of the structure may not be sutf- 
ficiently thinned for study. 
The usual practice has too often been to watch only 
the more prominent outlines of structure, to thin the 
section until these are as transparent or translucent as 
possible, which generally means to make the section as 
thin as possibie before stopping. But by this time all 
15 
