NOVEMBER 3, 1899. ] 
lenses. The images which they will form 
in space will be reversed, that is the lens X 
will give an image* in which B will be to 
the left of A, or just the opposite of the ap- 
pearance presented when the eye is at X. 
It is apparent that the images BA and AB 
formed by the lenses are identical with 
what would be seen by eyes at X and 1X’, 
provided A were in front of B, consequently 
the fusion of these twoimages makes A ap- 
pear nearer than B. 
It is possible for one who has trained the 
eyes to view stereoscopic photographs with- 
out the aid of the stereoscope, to bring the 
two images together in the same manner, 
but most persons will require the assistance 
of the prisms. My instrument consists 
simply of a double magnifying glass (con- 
sisting of two lenses mounted in rubber 
frames) mounted on the picture holder of 
an ordinary stereoscope, as shown in the 
figure. 
A neater device would be two small 
lenses cut square and mounted in a frame 
arranged to slide along the bar of the stereo- 
scope, or better still the instrument could 
be given the opera-glass form. 
The best objects to view with the instru- 
ment are small decorated bowls either right 
side up or bottom up, and such simple ob- 
jects. They appear to the best advantage 
when viewed from above. 
The image appears reduced in size but 
exceedingly brilliant and sharp and the 
pseudoscopic effect is sometimes perfectly 
startling. 
If the experiment is tried in the manner 
which I have described with a double 
magnifying glass, it is important to see that 
the two lenses have the same focal length, 
which is often not the case. 
R. W. Woop. 
Mapison, Wis. 
*T have drawn the images formed by the lens erect 
for the sake of simplicity. They are, of course, in- 
verted in reality. 
SCIENCE. 
649 
THE SUBDIVISION OF GENERA. 
In view of the almost universal accept- 
ance of the doctrine of evolution by natur- 
alists, most of the old discussions regarding 
the ‘ generic value of characters’ read much 
like those about the nature of phlogiston. 
If we must admit that even species are 
largely conventions, holding good only so 
long as our observation of them is limited 
in respect to time, areas and conditions, the 
larger subdivisions—genera, orders, etc.— 
must necessarily and a fortiort be regarded 
in the same light, as groups comprising 
forms agreeing in a large number of im- 
portant and striking characters, and which 
it is, therefore, convenient to regard from a 
common standpoint for the purposes of 
study; as forms probably derived from a 
common ancestor at a relatively not very 
remote time. If then admittedly genera 
are not established by nature but are man- 
made, it would seem desirable to adopt 
with reference to them the policy most con- 
ducive to a ready and comprehensive view 
of their relations to other groups of forms, 
and to facility of study. The latter con- 
sideration should weigh heavily, in view of 
the steadily increasing interest and instruc- 
tion in natural science. It is certain that 
the study of the latter is greatly hindered 
by the multiplication of names, both generic 
and specific, and by the unnecessary sub- 
stitution of terms of Greek and Latin deri- 
vation for well-understood English words 
of definite meaning. 
On the other hand, the detailed study of 
any group by specialists necessarily results 
in the discovery of new common characters 
within certain closely-related groups of 
forms, by which they may be conveniently 
subdivided for comparative study. Of 
course, there can be no question of the im- 
portance of such study of the minute char- 
acters, which leads us more and more 
closely to the immediate effects of environ- 
ment. The only question is how best to 
