7 
272 Rev. W. Hodgson’s Description of the Chronoscope. 
the instrument described in this paper, the planes are so ar- 
ranged that an image produced by ¢hree internal reflections, 
is brought to coincide with an image arising from éwo such 
reflections ; and when good flint-glass prisms are used, the 
loss of light is much less than would be imagined by those 
who are not practically familiar with the phenomena of inter- 
nal reflection. The image of three reflections may also, in 
this instrument, be compared with the unreflected object, but 
then, in order to secure exact coincidence of the image and 
object, the instrument requires to be placed (as is necessary 
for the sextant, reflecting circle, and dipleidescope) in such a 
manner, that the rays of light are incident upon the reflectors 
in a plane perpendicular to their intersection. With the 
images arising from two and three reflections this is not es- 
sential; for as parallel rays, incident upon the prisms, are 
symmetrically reflected and refracted, they emerge parallel 
to each other even at great obliquities, and are free from 
chromatic confusion. 
In the case above referred to, in which the faces of the 
prisms, instead of being placed in contact, are inclined to 
each other at a small angle, if the instrument is fixed so that 
any coincidence of the solar images occurs when the sun’s 
centre is on the meridian, the azimuths of the other planes in 
which this phenomenon takes place may be easily found ; and 
from these, the sun’s declination and the observed time of his 
passing any one or more of these planes, the /atitude of the 
place may be readily computed. 
In default of a more appropriate name for this simple in- 
strument for finding the time of observation, and thence de- 
ducing the latitude and longitude of the place of the observer, 
the term CHRONOSCOPE is suggested by the inventor, who has 
the honour to be, Sir, your obedient servant, 
WILLIAM Hopeson. 
~~ a 
