INSTRUMENTS FOR LOOKING THROUGH THICK WALBS? ory)... 
WITH SMALL APERTURES. eee 
BY 
A. H. RUSSELL, Caprarn or Orpnancz, U.S. Army. 
COMMUNICATED BY 
THE SHCRETARY. 
In this paper, where a lens is spoken of, any combination of lenses is 
meant between two conjugate foci. Jor instance, the arrangement 
shown in figure 7 constitutes one lens in the sense in which the term 
is used, and by such a combination a greater range of vision can be 
obtained for equal distances between apertures than by a single lens of 
the same size. 
Armine Guns THrover Tick Watts or Forts anp TURRETS. 
The instruments here described afford means of sighting through 
the thick walls of a fort or gun turret without the use of a wide open- 
ing, exposed to shots from the enemy. The outer aperture in the wall 
may be a mere pinhole or an extremely narrow slit. 
Those mentioned in Part I., are purely optical, employing fixed 
lenses or fixed mirrors; while those in Part II. are mechanical as well 
as optical, employing parallel plane mirrors, one or both being movable. 
The mechanical contrivance allows also modifications which result in 
a measuring instrument having four times the accuracy of the sextant; 
while the optical trisection of angles can also be made with ease. 
PART T. 
Instruments with stationary lenses or mirrors, giving a wide angle of 
vision through small apertures. 
Ist. Refracting Instrument. 
By setting a lens Z (Figure 1) before a thin diaphragm having a 
Fre. 1. 
pinhole perforation P, and placing the eye at the point J where rays 
SJ, \/Olo SxSatit, 53 
