32 THE RATTLESNAKE AND NATURAL SELECTION. 
the substage removed and the body racked down so as to foc 
through the empty stage upon the table, a block or box, or an 
temporized stage occupying the usual position of the mirror i 
illuminated by the mirror after the method suggested by Mr. 
James Smith. In this case it is desirable to increase the wors 
distance between the prism and the object by varying the le ns 
employed. Thus a one-and-a-half-inch objective at from th 
and three-fourths to five and three-fourths inches from the ere 
will give powers of six to fifty diameters and working distan 
from prism óf seven to ten inches. The erector may also be 
moved an inch or more from the prism. When this latter arran 
ment is to be used exclusively, placing the object at from eig 
to ten or twelve inches from the prism, as in many students’ 
croscopes, the apparatus is further simplified by screwing 4 4 
inch objective into the nose-piece in its usual position, as an ¢ 
tor, and screwing or sliding over it an adapter carrying a-one 
a-half or two-inch Bbjeditve from four to six inches lower dow 
Some contrivance is required to illuminate transparent objects t 
der the lower powers ; but opake and translucent objects on a bl: 
ground can be dissected and manipulated with great facility. 
The same erecting arrangement can be used in connect 
with monocular microscopes that have no draw-tube and theret 
cannot use an erector in the usual position. It may also be u 6 
as a means of working Wenham’s and other binoculars, ¥ 
high powers. With powers of five hundred or one thousand 
ameters, however, it is still difficult to obtain good definition or 
fully light both fields. 
’ 
THE RATTLESNAKE AND NATURAL SELECTION 
BY PROF. N. S. SHALER. 
For some years I have been teaching that the tail appendage 
= rattlesnake was not to be explained on the doctrine of 
, inasmuch as it could contribute in no way to the £ 
uk of the animal. It seemed to me quite clear that it was 
calculat d to hinder than to help the creature in the race of 
by warning its prey of its presence. Nor did it seem easy t0 
