Marcu 18, 1915| 
NATURE 
69 

ceed far enough down the tube to be received by 
the second telescope, and so transmitted to the 
eye.) 
In modern submarines the tube has a length 
of from 16 to 24 ft., the diameter is from 6 to 
9 in., while the field of view is about 65°. In 
order that objects shall look their real size, it 
is necessary to give a magnification of one and a 
quarter to one and a half. 
Fig. 3 gives an illustration of a periscope in 
which three telescope systems are employed. The 
drawing is made from information published by 
Messrs. Goerz! of Berlin, and relates to peri- 
scopes made by them. It is, of course, undesir- 
able to give any details of English periscopes at 
the present time. 
An outer tube has a spherical glass cover. In 
the inner tube is the optical.system, which can be 


rotated to face 
in any required 
direction; the 
eye piece, how- 
ever, remains 
fixed. 
The optical 
system, = which 
follows in its 
general prin- 
ciples Sir How- 
ard Grubb’s 
original design, 
consists of :— 
(1) A reversed 
telescope, giving 
a reduction of 
about one quar- 
ber; ; 
(2) PA teller 
scope, giving a 
magnification of 
about two; 
(3) An _ erect- 
ing prism which 
can be rotated 
SOM uthiatis thre 
image given by the system is correctly oriented; 
(4) A telescope giving a magnification of about 
three. 
This telescope includes a fixed eye piece and 
prism, so arranged that the observer looks hori- 
zontally at the object. At the focus of the eye 
piece are placed a scale and pointer to show the 
bearing of the object sighted, and a ruling to 
allow the distance to be estimated when the size 
of the object is known. 
By the aid of a subsidiary system, special parts 
of the field can be further magnified to allow of 
objects being examined in more detail. 
The continued use of the periscope is very 
trying to the eye, so that devices have been used 
to throw the image on to a ground glass screen. 
The ordinary eye piece and ground glass systems 
are made interchangeable, so that the observer 
can readily pass from one to the other; he may 
4 Dr. Weichert, Jahrbuch der Schiffsbautechnischen Gesellschaft, 1914. 
NO. 2368, VOL. 95] 


Upper Part. 

Fic. 3.—a, Glass cover; 4, prism; c, d@, and e, 
lenses of reversed telescope :_/ outer tube; g, 
inner tube; /, 7,.and 7, lenses of second tele- 
scope; &, reversing prism ; 2, 7, and 2, lenses 
of third telescope ; 4, prism; 7 and s, pointer 
and scale; ¢, rotating mechanism. 

observe with the ground glass in the ordinary 
way, but examine special objects with the ordinary 
eye piece. 
The field of view of the periscope is still limited, 
and various attempts to overcome this difficulty 
have been made. More than one periscope can 
be used and the images combined to form a com- 
plete image. A recent improvement consists in 
the use of a ring reflector which enables a view 
of the whole horizon to be obtained at once. The 
image formed by the ring system is much dis- 
torted, but when any object is picked up it can 
be examined by means of the ordinary system. 
These two optical systems are combined in one 
instrument, so that the two images are seen in 
the one field, the image formed by the ring system 
surrounding the other. 
But these ring periscopes are still far from 
perfect, their distortion making it very difficult 
to identify objects; and this difficulty, though not 
so pronounced, occurs with the ordinary periscope. 
The point of view from which the surface of the 
sea and surrounding objects are seen is one to 
which the eye is not generally accustomed. The 
conditions of lighting, too, render it difficult to 
distinguish objects, especially when there is mist 
or spray, so that the effective use of a periscope 
requires considerable skill and training. 
Trench periscopes may be obtained from most 
opticians, and the following are a few typical 
forms :— 
The Hampson, wooden 
mirrors; price 7s. 6d. 
The Adams, jointed rod; price 10s. 6d. 
The Stanley; the support is in the form of lazy 
tongs, and is of a light alloy; price 25s. 
These open-mirror types are light, portable, 
with good field, but the mirrors are not protected 
from rain, and the useful field is surrounded by 
bright sky. 
Tube types are made by Messrs. Negretti and 
Zambra, Dixey, The Periscope Co., and many 
others; prices from 8s. 6d. to 15s., depending on 
the metal tube used. In these types the field is 
rather limited. 
Messrs. Chas. Baker and Co. supply a type 
with large mirrors, which can be satisfactorily 
used with both barrels of a binocular. In spite 
of the large mirrors, the type is very portable; 
price 30s. 
Many makers supply types in which the 
optical system is incorporated with the periscope, 
and the prices of these range from 28s. to 61. tos., 
according to the type of optical system used. 
S. D. CHALMERS. 
stake carrying two 
OIL OF VITRIOL AS AN AGENT OF 
* COEUR 
We 
a former article under this heading (NATURE, 
December 31, 1914, vol. xciv., p. 472) we 
pointed out that Germany’s ability to continue the 
war depended largely upon her power to maintain 
her supply of oil of vitriol, this product being 
absolutely indispensable in the manufacture of 

