68 
NATURE 
[ Marcu 18, IQ15 

omitted, so that h reduces to the forms already 
eiven when viscosity is neglected altogether, F 
being again a function of a_ single variable, 
avc/K or.buc/x. In any case F is constant for 
a given fluid, provided v be taken inversely as 
a or b. RAYLEIGH. 

PERISCOPES. 
HILE the periscope of the submarine is 
developing in the direction of greater 
optical perfection and elaboration, there has been 
a return to the simplest and earliest types of 
periscope for use in land warfare. Some of these 
trench periscopes recall the polemoscope, described 
by Helvelius in the seventeenth century for 
military purposes; this polemoscope in its sim- 
plest form consisted of two mirrors with their 
reflecting surfaces parallel to each other, and 




Fic. 1. 
inclined at 45° to the direction of the incident 
light. These mirrors were mounted in a tube and 
separated a convenient distance (Fig. 1). 
For modern trench warfare the convenient 
separation is about 18 to 24 in., and the mirrors 
are mounted in tubes, in boxes of square or oblong 
section, or attached to a long rod. In each case 
it is necessary that the mirrors should be fixed 
at the correct angle, and that there should be no 
doubling or distortion of the image. 
The principal requirements of these trench peri- 
scopes are portability, lightness, small size and 
inconspicuous appearance, and large field of view. 
When there are no lenses the field of view is 
exactly the same as would be obtained by looking 
through a tube of the same length and diameter. 
Thus, with mirrors of 2 in. by 3 in. and a separa- 
tion of about 22 in., a field of view of 5° would 
be obtained; and by moving the eye about, this 
field could be nearly doubled. 
NO. 2368, VOL. 95 | 


By using a box of oblong section the horizontal 
field of view can be increased without unduly 
increasing the size of the periscope. As the field 
of view is somewhat limited in any case, the prin- 
cipal objection to the use of a telescope or bino- 
cular, viz., the reduced field, no longer applies, 
and many periscopes are arranged to be used with 
a monocular or a binocular telescope. 
Most periscopes can be used with a magnifica- 
tion of two or three, i.e., with one tube of an 
ordinary opera glass; but when higher magnifica- 
tion is to be used the mirrors must be of better 
quality, both as regards flatness of surfaces and 
parallelism of the glass. When the mirrors are 
large enough—8 to 10 centimetres wide—both 
telescopes of the binocular may be used, but in 
this case the requirements for the mirrors are even 
more stringent, as the images formed by the two 
telescopes will not coincide unless the mirrors are 
plane. When suitable lenses are placed between 
the mirrors, the size 
of the mirrors can be 
reduced or the field of 
erect image, or mir- 
rors or prisms 
ployed to erect the 
image. An example 
of a periscope of this 
type is shown in Fig. | 
em- 
—$—$<—> 
view increased; it is | 
easy to provide a small 
magnification of the a 
image or even to ar- | 
range for a variable 
magnification. | 
In such cases the a 
lenses must be = ar- | 
ranged to give an | 
| 
| 
| 2, where the mirrors ° 
are replaced by reflect- 
ing prisms, and the 
prisms erect the image in much the same way as 
the prisms of a prism binocular. 
This arrangement is very suitable for a large 
magnification, but for larger fields the prism 
is unsuitable, unless it be silvered, and it is 
preferable to erect the image by means of 
lenses. 
When longer tubes are used. or larger fields 
are required, the design should approximate to 
that used in the submarine periscope. 
This optical system has been steadily developed 
since its first introduction by Sir Howard Grubb 
in 1got. 
The system consists of two telescopes, of which 
one is reversed, so that the image would be 
reduced in size, while the other magnifies this 
image, so that the final image is of the same size 
as the object, or is magnified one and a quarter 
or one and a half times. (As a very large angular 
field of view is required in these periscopes, the 
beam reflected into the tube must cover a large 
angle, and would soon fall on the sides of the 
tube; the reversed telescope, however, reduces 
the angle of the beam, and so enables it to pro- 
Fic. 2. 
