136 
THE YOUNG SCIENTIST. 
To See Objects Apparently through the 
Hand. 
THE following curious experiment 
throws a good deal of light upon the 
facts connected with vision. 
Take a sheet of ordinary letter paper, 
roll i»t into a tube, and through it look at 
some distant object with the left eye, the 
right eye being also open. If the hand 
be now held before the right eye, so as to 
cut off the view of the object by that eye, 
the object will still remain visible to the 
left eye. Strange to say, however, it will 
appear to be visible to both eyes, and a 
hole, the exact size and shape of the tube, 
will appear through the hand as shown 
LOOKING THEOUGH A HOLE IN THE HAND. 
by dotted lines in the engraving! In 
other words, it will appear to us as if 
there was actually a hole through the 
hand, the objects being seen through that 
hole. 
This singular optical illusion is evi- 
dently due to the sympathy which exists 
between the two eyes from our habit of 
blending the images formed in both eyes, 
so as to form a single image. The result 
is startlingly realistic, and forms one of 
the simplest and most interesting experi- 
ments that we know of. 
Mr. Wen ham's Paper on Object-Glasses. 
^^HOSE who read these papers will see at 
-L once that some of them are intended for 
any but "young" scientists. Indeed, it 
will require a very intelligent student to 
follow the one in the present issue, and we 
would have omitted this, as unsuited to 
our columns, if it had not been that to do 
so would have left the series incomplete. 
We have thought it best to give Mr. Wen- 
ham's papers entire; a large portion of 
them are so plainly written that any in- 
telligent boy can understand them, and 
the rest is of value to such a number of 
our subscribers that the space can not be 
considered wasted. 
BOOK NOTICES. 
The Students' Illustrated Guide to Prac- 
tical Draug-htingr. A Series of Practical 
Instructions for Machinists, Mecliaiiics, Ap- 
prentices, and Students at Engineering- Estab- 
lishments and Technical Institutes. By T. P. 
Pemberton, Draughtsman and Meclianical En- 
gineer. Illustrated with Numerous Engra- 
vings. Price $1.00. New York: Industrial Pub- 
lication Co. 
This is a simple and thorough book by a 
draughtsman of twenty-flve years experience. It 
is intended for beginners and sell-taught stu- 
dents, as well as for those who pursue the study 
under the direction of a teacher, and having been 
written by one who has had a large experience in 
teaching, the wants of both classes have been very 
fully met. So far as regards the mere art of draw- 
ing, the advanced student will probably find little 
in this book that will assist him, but the chap- 
ters relating to gearing and screws, aim to give 
thorough and practical knowledge, not only as 
to the best methods of delineating them, but as to 
the true principles involved in their construction. 
In these departments the author has been as- 
sisted by the well-known mechanicians, Joshua 
Rose and John Walker, whose names are a suf- 
ficient guarantee of the excellence of the methods 
explained. 
Levers of Archimedes, with the Fulcrum 
Found. By Will Powers. Philadelphia: G. W. 
Townsend. 
These "levers" consist of a certain system of 
examining and studying any subject that may be 
proposed. The points to which special attention 
is to be directed are the cause the essentials, the 
associations, etc., of the subject to be studied, and 
the so-called levers " are printed blanks in which 
this scheme is fully laid out. 
The idea is a good one; whether the "levers" 
will enable the student to carry it out or not is, 
